I am a British blogger, who lives in Japan... and I follow a lot of motherhood-related stuff in the States, because a lot of my favorite blogs and parenting groups/forums are US-based. There are lots of differences in breastfeeding culture between the three countries that one could potentially talk about--differences in attitudes to supplementation, newborn procedures, cultural oddities like beliefs about alcohol and diet while nursing--but one aspect where one sees big differences is attitudes towards pumping and bottlefeeding expressed milk.
Breastfeeding advocacy has always had an ambiguous, push-me-push-me attitude towards expressing--is it a "good" thing that replaces formula in bottles, or is it a "bad" thing that competes with the act of nursing directly at the breast? You might imagine that cultures which make it easy to nurse your baby directly will also be places that make it easy to pump and bottlefeed your breastmilk. In fact, that's often not the case, as we shall see. A quick disclaimer--the following discussion is mostly based on my personal experiences of these breastfeeding cultures both online and in real life, so this time I don't have a lot of data to back me up. If anyone does have any data that disproves anything here, speak up!
United States
My "main" mums' board when Baby Seal was tiny was predominantly American, and one thing that stood out was the amount of time spent talking about pumping. The United States is almost unique among developed countries in that maternity leave is still not considered standard; unless you are exiting the workforce for a bit, you will probably be back at work about six to eight weeks after giving birth. For US breastfeeders, therefore, pumping is a continual source of conversation, commiseration, competition and anxiety, and US-centric breastfeeding blogs and pages tend to spend a lot of time talking about women's pumping rights at work.
To a certain extent, the normalization of pumping and bottle-feeding EBM has bled over into the experiences of stay-at-home mothers in the US, who often sock away quite substantial amounts of milk in the freezer "just in case," or with the hope of donating it at some point. The custom of carrying bottles of expressed milk around in public for feeding the baby with also seems to be commoner in the US than elsewhere, probably because public breastfeeders are more likely to get hassled (although, like everything else American, this is very regional). Exclusive pumping seems to be commoner in America too--on British fora like Mumsnet, women who want to feed their babies this way are often advised to look on American fora and Facebook pages for advice. Perhaps widespread car ownership also tends to make exclusive pumping a bit more doable in the US than elsewhere, because you can pump in the car and because a car makes it easier to tote bottles, ice packs and heavy-duty breast pumps around with you. A few women even opt for exclusive pumping due to personal choice.
Japan
Complete contrast here. Although you won't actually see many women nursing publicly in Japan, Japanese culture seems to be far more accepting of the "nursing relationship" than the States. But while Japanese culture is fairly nursing-friendly, this does not extend to pumping. You can get breastpumps in Japan, of course, and increasing numbers of women use them. But relatively few women work outside the home when their babies are small, due to maternity leave and the fact that so many women leave the workforce for many years or permanently once they have children. Using babysitters also seems to be relatively uncommon.
As a result, the culture of pumping and bottle-feeding EBM has not become rooted in Japanese culture in the way it has in the States. Quite a lot of women do give the odd bottle, but more often than not there will be formula in the bottle--a fact facilitated by the fact that Japanese mothers seem less likely to place a strong premium on breastfeeding exclusively. It's the same story with the pumping side. If you are one of the minority of women who goes back to work early, don't expect much provision or understanding for your pumping needs if you want to pump. I mostly work from home, but was asked to take on an on-site once-a-week position when my daughter was six months old. When I nervously broached the subject of pumping, they were nice enough but said that there was nowhere in the company where I could express milk. Given that the company in question consisted of a 37-storey megablock, I found that hard to believe--but it was clear that my prospective employers were having difficulty envisaging what I was asking for, because they had never received such a request. In the end, I turned down that particular job for unrelated reasons, but it was an awkward moment.
And then there's the question of getting milk into the baby. Like many things in Japan, the process of sending EBM to daycare is based on lots of silly rules and the most unbelievable inefficiency. In theory, public daycares are supposed to accept EBM; in practice, many of them outright refuse, or put enormous pressure on parents to send formula. If they do accept EBM, the majority insist on the milk being frozen first. Some daycares further stipulate that the milk has to be frozen BUT also pumped within the last week (which means that you basically have to pump, freeze the milk, and then almost immediately dig it out of the freezer to thaw it in the fridge). Apparently, even NICUs often insist on using frozen EBM rather than fresh. I have no idea why the Japanese are so obsessed with freezing EBM--are they under the impression that the freezing process "kills germs" or something? It doesn't, of course (and thawed frozen breastmilk is actually more prone to spoilage than milk that's never been frozen, because the freezing process zaps some of the natural microbicidal compounds). Given the critical shortage of daycare spots in big cities in Japan, it's unlikely that we'll see women demanding that daycares change these ridiculous rules any time soon--most women who have got a spot are too busy feeling relieved and grateful to raise many complaints, and content themselves with writing fake dates on their bags of frozen milk.
Britain
In Britain, as in America, it would be pretty much unthinkable for a daycare to refuse to handle expressed breastmilk. On the other hand, British women, like their Japanese counterparts, tend not to work full time when their babies are young because most take maternity leave. So pumping has never made quite the inroads into British culture that it has in the States (which is perhaps why the slightly awkward word "express" seems to be the commoner verb to use), although the majority of breastfeeders do own a pump and use it now and again. On British (and Australian) discussion fora, quite a lot of breastfeeding advocates encourage women not to make any use of pumping and bottlefeeding at all, on the grounds that conveying milk this way is unnecessary, might mess up the mother's supply and/or create a preference for artificial teats in the baby, and is less healthy/optimal than direct breastfeeding. I think it would be a rare American lactivist who was so negative about pumping and bottlefeeding; so many American breastfeeders work full-time that she would risk alienating a large percentage of her target audience, and if you overemphasize the difficulties of maintaining breastfeeding through pumping and bottlefeeding to mothers who are going to WOH anyway, they might just give up and decide to use formula instead.
Conclusion
Living in Japan, it's certainly easy for me to be irritated by aspects of the culture which are interconnected with the difficulties of expressing and bottling breastmilk. Japanese culture tends to emphasize the importance of the mother-child bond; this can be liberating in some ways (very few people here will look down on you for breastfeeding a toddler or even letting them share your bed), but it's also connected with the fact that Japan has been very slow to get women into the workplace--something which is now a big problem for Japan given its shrinking workforce and its resistance to accepting large-scale immigration. The cultural squeamishness towards breastmilk that's been separated from the mother's body and the lack of legal protection for pumping at work means that many women are not able to pump their milk or feed it to their babies if they want, which deprives them of choice; others (as outlined above) are forced to go through unnecessary and time-consuming procedures which add to their burden. While maternity leave provision is a good thing, I think we should also recognize the fact that some women want to return to work while their babies are small--and they have every right to do so and leave pumped milk if they wish. I don't think that the cultural resistance towards using babysitters does Japanese marriages any good, and I'm not a fan of seeing cranky babies being toted around to smoky restaurants late at night, as is frequently seen here in Tokyo.
On the other hand, normalization of pumping can also become a trap in its own way. There is a danger that each technological innovation can become something that merely shifts the goalposts of what mothers are expected to be able to achieve. Since breastpumps have been invented, women have increasingly been expected to combine breastfeeding and working outside the home (whereas their mother's generation, if they worked outside the home when their babies were small, just shrugged their shoulders and used formula); more efficient breastpumps have raised the bar higher, with working women increasingly expected not only to "breastfeed" but to "breastfeed exclusively without formula supplementation"; with bigger freezers, better designed storage containers and more affordable double-electric pumps, even women who are not working outside the home may increasingly feel that it is expected and normal to pump "just-in-case" stashes or donate milk. And writers like Olivia Campbell may have a point when they suggest that this technological fix (ever higher-performing breastpumps) has enabled American society to avoid confronting the issue of maternity leave, opting instead to place a double burden (working and pumping) on the shoulders of fragile postpartum mothers. Similarly, because most people in the US are aware that pumping and bottle-feeding EBM is a "thing," this can lead to more pressure on mothers to carry bottles of pumped milk around with them, and can become an excuse for negativity towards women who are trying to nurse their babies in public ("Come on, lady--they invented breastpumps for a reason!! Why don't you just pump a bottle before you leave the house??").
Considering each of the above countries in turn, I think my own country--the UK--probably represents the best overall balance for respecting women's right both to feed at the breast and to pump when they need or want to--although I do wish that there was a bit more awareness in the UK of the fact that bottles are not the only way to get breastmilk into a bottle-refusing baby. I hope that each of these countries can work towards achieving a cultural consensus that will allow women and babies to take advantage of the best aspects of both direct nursing and pumping/bottlefeeding--without either becoming something that places unnecessary burdens on women or restricts their freedom.
Further reading
Baby Food: If breast is best, why are women bottling their milk? (Jill Lepore, New Yorker)
Why I pump and why you shouldn't feel bad for me (Healthy Tipping Point)
The Unseen Consequences of Pumping Breast Milk (Olivia Campbell)