First, not everyone in the US is using all these current Briticisms. I suspect they're entering the language by different routes. The route that's most clear in the examples that Yagoda gives in his blog is northeastern media/publishing. When writing about Yagoda's blog, I said:
...some of the BrEisms that Yagoda picks out as "widely adopted" strike me as not so. For one thing, some of them are things that Americans have sent me puzzled emails about. For another, the sources Yagoda cites are very often New Yorkers, if not The New Yorker, and most come from the NY-DC corridor. [...] I'm having a hard time finding out how many of the 685,000 British expats in the US are in New York, but many commentators seem to agree with A.A. Gill that "The British have colonized Manhattan". And an awful lot of them seem to be in publishing. So, it could be a trend in a certain milieu. [...] I'm not saying that all the BrEisms are coming from UK expats; I have no trouble believing that Americans in their milieu are easily influenced by chic-sounding British words. And if that continues, those words may make their way into general American English. But my impression from non-NYCers is that these words are far from "widely adopted."
- They fill a gap.
- They sound 'cool' to someone for some reason (e.g. they sound intelligent, exotic)
- Most people aren't really aware of the origins of the new word, and so don't care that they've adopted a Briticism. It's just a new word to them.
Fewer people negatively judge the borrowing of words in situations (1) and (3). Some of the past BrE-to-AmE WotYs are in situation (3), for example go missing and to vet. While Americans are often bad at knowing which words are Britishisms (many Americans seem to believe that bumbershoot is an English way of saying 'umbrella'), the British are probably worse at knowing which are Americanisms.*
But in case (2) the judg(e)ments come swift and hard. The US press is referring to it as Anglocreep. The UK press mostly just calls it [insert pejorative adjective here] Americanisation.
People find situation (2) threatening for a number of reasons--all to do with our sense of language as a marker of identity. If you're using words from a different place that you don't have 'birth rights' to, you're seen as 'inauthentic' in the use of those words. You can also be seen as rejecting the language, and therefore the identity, of the people and place that you come from. Taking on those new words also marks you as aspiring to be associated with a group of people who may not always be positively stereotyped in the culture you're in--and those stereotypes rub off on your word usage in convoluted ways. So, taking on American words is seen as 'sloppy' and 'lazy' in the UK. Taking on British words is seen as 'snobby' and 'pretentious' in the US.
Another reason why people complain when their words are borrowed by others is that they're rarely used in the new place just as they were in the old place. The pronunciations, of course, are adapted to the local accent, but the meanings of the words often change also. This is true of all borrowings. We don't use the word spaghetti like Italians do (for them, it's plural) nor douche in the way the French do. But the differences are more glaring when it's borrowing within the same language and we're all trying to use the language to communicate with one another, which involves assuming that we're using the words in the same way. The social significance of words (particularly how offensive they might be considered to be) changes a lot--and sometimes nuances of meaning are missed. Some examples:
Americans are notorious for using and not understanding the connotations of wanker (see the comments in that post for some stories). Americans imported wanker without necessarily knowing wank (to masturbate), and so it sounds like a fun, silly thing to call people. But calling someone a wanker is less like calling them a jerk, and more like calling them a jerk-off. In the other direction, we've noticed British students coming back from a year abroad in the US and using the youthy use of douche as an insult, but in social contexts in which my brothers/nephew would avoid it in the US (the family dinner table, with grandma. OK, ok, I'm talking about my brother-in-law).
Newcastle Brown Ale's No Bollocks ad campaign is specific to America and it's not clear that such a campaign would be allowed in mass media in the UK. The Advertising Standards Authority's 'Deleting Expletives' [link is pdf] report of 2000 put bollocks as the 8th most offensive word according to the British public. (Wanker was 4th, before nigger or bastard.) Words lower in the 'severity of offence' list than bollocks include arsehole, twat and shit. Having typed these words, I have now guaranteed that my blog will not be readable from any school computer anywhere. But anyhow, the facts that (a) you can use this word on a billboard in the US and (b) someone has done so pretty much guarantees that the word is being used in the US in ways that it wouldn't be used in its native country.
I've noted before examples of Americans using BrE expressions with distinctly non-BrE meanings, for instance snog and chat up. One I came across yesterday was an AmE speaker using BrE starkers (which means AmE barenaked) to mean 'crazy', having been misled by another BrE phrase, stark raving mad. There's more potential for that in the BrE-to-AmE direction, I think, because the pathways the words are travel(l)ing are narrower than the ones that go AmE-to-BrE. But there are still BrE uses of AmE words that are unlike the original meaning. I've talked about this before with reference to shotgun and I've known a few BrE speakers who've assumed that a raincheck is a refund.
I'm looking forward to whatever else is to come in the media discussion of BrE words in AmE places--and I'll try to remember to link to them here. Till then, (BrE-to-AmE, not without controversy) cheers!
* I have lots of examples of this in a talk I've been giving a lot lately: 'How Americans Saved the English Language'. If you'd like to hear it, all you have to do is have your local speaking club invite me at a convenient time and pay my expenses to get there. At this point, the next one is Lewes, East Sussex in December. I'll give details closer to the date.