I don't think that's true at all. Australians are friendlier than most on a one-on-one level and Americans are just as big campaigners on the road as anywhere else.
Nope, the US has a built-in culture of politeness that we don't have. Like Power Raid says, though, it's a veneer. Generally when we're polite to someone it's because we have a genuine interest in them, which Americans don't.
I don't think it has any impact on roundabouts vs stop signs though. To me that just seems to be what each country has been brought up on, and now it's habit.