Every culture makes a public/private distinction and has different social conventions in each.
For example, sarcastic/ironic/cutting humour is mostly a private-space thing in america -- whereas in the UK it's common to both. I'd say this is related to their deep sense that public spaces should be friendly-distant.
For this reason, amongst others, Americans have a reputation for not understanding irony/british sense of humour. It's more that they don't expect it in public places. I'd imagine an American experience of people British people joking at (say,) a funeral is something like a British experience of an American helpline. Vaguely repellent.
It's because it's mixing public/private which crosses intimacy lines. And intimacy is closely related to disgust (eg. someone you dont like kissing you).
PS. I've just had another thought about that. One of the issues we have in the UK at the moment is the importation of a strange kind of political correctness where you mustn't joke about stereotypes etc. (which is a particularly european thing: to joke about each other's culture -- this tends to feel connecting to europeans).
My guess is that because, in America, public spaces are supposed to be friendly-distant (hence why they censor all sorts of words, etc.) we have imported this sense about certain kinds of jokes.
To the American sensibilities a stereotyping joke in a public space is alienating. To a european one, it's a gesture towards intimacy.
I think we're probably going to lose this to American culture. We may have already.
Though still, it's interesting to observe: get a group of europenas of different nationalities together and they bond by joking about sterotypes. Stereotype an american and they think you're being hostile and trash-talking them.
This maybe due to the physical boundaries of Europe, that it's no insult to comment on another's "otherness" when they are traditionally (and a probably self-acknowledged) outsider. In the US, we're all stewing in the same pot with a more than a dash of (at least historical) racism and bigotry, so (negatively) emphasizing differences goes against our cultural mythology.
I can assure you, Americans have no difficulty with ridiculous caricatures of sterotypical British/French/German/Italian/etc behaviours.
That's the missing context British people have when opining about the American way of doing politics: that it's keeping a lid on something that here we do not have.
However I think it's more than that. The places that have the most european sense of humor and way of live are those with the highest population density. New York / Boston humour is quite close. Yet, NY -- the most dense US city -- is still only have as dense as paris.
The US is incredibly sparse. Everything is very far away from everything else even within cities: a shop is a drive from another shop.
I suspect that does something peculiar to public spaces.
The recent civil war and racism doesn't help though.