As has been pointed out by several other people on here, "rudeness" isn't exclusive to Americans, by any interpretation of the word. For example, there's nothing "rude" in assertively (yet politely and calmly) insisting on good customer service. Even Brits (Michael Palin and John Cleese immediately come to mind) will joke about how they could be at a restaurant, with horrible food served by an awful waiter, complain about it amongst themselves, yet when asked how everything is going, say it's great. To state otherwise would be considered rude; that's not rude--it's the rational assumption that you should get what you pay for. I've also heard many Brits complain about German tourists hogging poolside lounges, jumping queues (or butting in line, as Americans say), etc. That, to me,
is rude. But when people make blanket statements that "Americans are rude" (each and
every one of the approximate 290 million of us), the sheer statistical improbability of that is enough to make me bite my tongue and not say something
truly rude in reply.
If some of the behavior I've seen in Brighton's city center on weekend nights, or from British fans at football games is "polite," then I suppose "rude" needs a complete change in definition in dictionaries worldwide.
I think it really just boils down to Americans' tendency to be straightforward--again, a different concept altogether from being rude.