Hmm, as a Canadian I can say that my experience of the stereotype has been that ingrained politeness is considered part of our national character; the impulse to apologize when someone bumps into you, for example. It's like drinking beer, loving hockey, Saying "eh," and wearing toques: stereotypically Canadian. It's not something that's used to put down other cultures, by saying we are more polite than other countries or cultures, or that other cultures/countries are rude. Our Canadian identity exists independently of the USA, and saying "Canadians are <insert broad generalization about Canadians>" doesn't mean that people from other countries can't also be <broad generalization>. So when I say "Canadians are polite" I certainly am not implying that Americans are not. I'm surprised that I can't ascribe a positive attribute to Canadians without people assuming that I must be insulting Americans because I don't also specifically say "but not more polite than Americans! Americans are also polite!"