Well, I've never really stereotyped Americans (apart from some good natured ribbing), so I guess I'm exempt. However, you can't deny that Americans do it to. The amount of shit I got in America for being Irish, every single person thought it was funny to mention wife-beating. I don't exactly know why, but for some reason, everyone I met in America who I didn't know seemed to be under the impression that all Irish were brain-dead, bigoted drunks, and treated me as such. This wasn't in the middle of nowhere either, this was in Seattle. So, Americans can't be completely absolved.
Still, it's basic human nature. If there's a country, thousands of miles away from you, unless you're very interested, you aren't going to know much about their country except for a few basic archetypes. When this becomes a problem, is when you start treating people differently because of it.