Agreed. I've only met one American who's as pretentious about it, but it's almost always British people or other Europeans. It's even more annoying coming from non-Brits. I had a student once on the first day tell me that she only wanted to learn British English. Welp. Sorry, you've got an American teacher, so good luck with that.
Now I've had much the same experience with preferences towards the British accent from non-English speaking European countries. I suppose if you couple that with Britpunk's reaction, and it starts to make sense. Certainly as an American I find pro-British chauvinism to be pretentious. I think I'd feel even more annoyed if I were British.
But its still pretty annoying as an American.