Oddly, Americans do tend to pronounce 'fillet' and 'niche' à la française. Except it's not odd, it just is.
I don't think that's particularly odd. Americans tend to think you're an uneducated oaf if you don't, so I'm sure they all learn quickly enough. Which makes for an interesting culture clash when they encounter the great British tradition of pronouncing words borrowed from French as Englishly as possible, presumably on general principle. Indeed, the rule seems to be that BRITONS are only allowed to use the French pronunciation when actually speaking French, lest they be perceived to be pretentious.
I find the whole thing somewhat amusing, even though it leaves me with the same sort of cognitive unease that I get when I can't decide whether to use a northern or southern 'a' sound (which is most of the time, as they both seem wrong and liable to result in piss-taking).
I'm not sure. If you're not speaking French, why make the effort to pretend, it always comes off a bit pretentious. I blame the BBC, it's payback for all the northern accents. Do Jesus and the Queen sound like they come from Manchester? No, and for good reason.
The French don't exactly try with English. They even the make that exquisitely gallic pouty-pouty poo face when they hear Québécois, like someone has been sick in their bouillabaisse.
Still, there is something a little bit special when the young person behind the counter at Mcdonalds corrects your 'fillet o' fish' like the most snooty of Parisian waiters. Oh you mean
fillet. It goes both ways though, many years ago in Texas I remember asking for the buffet breakfast. Cue blank look from the waitress. 'The buffet? said I, hopeful of overcoming our linguistic impasse through the medium of repetition. "Oh, you mean the bur-fit,' came the final spark of understanding.