This has been my experience as well. The superficial politeness is often a thin veil of insularity and self-righteousness. I often noted that these folks have an unshaken belief that America is an island of civilization in the see of barbarism and they treat "deserving" (i.e. those who adapted to the "superior" American way) outsiders with patronizing acquiescence. But that patronizing politeness becomes particularly annoying when one realize that these folks are absolutely resistant to any point of view that is different than their own. That leaves one with the choice of either being polite and nodding to that display of ignorance cum self-righteousness, or shatter that display and risk being seen as a rude barbarian - which would reinforce their feeling of superiority anyway.
I think that attitude is not limited to "middle America" - you can see that in NYC as well to certain extent, but what makes NYC different is it extraordinary diversity, which makes the babbitts at least aware of different cultures and points of view.
I need to add, however, that ethnocentrism is by no means a unique US phenomenon - there are countries much worse than the US in this respect. What make the US stand out is, for a large part, expectations of the observers. On one level, US has been advertised, especially abroad, as a "melting pot" and a "multicultural society" - so it is a bit surprising for an outsider to find the all-familiar bigotry underneath the advertised multiculturalism.
On another level, the expectation of broad mindedness and tolerance is linked to social class. One is not surprised when a redneck is a bigot, but that is not the case when a person of a higher social standing and education turns to be one. Since class identities are often blurred in the US, outside observers often misperceive the social class of their interlocutors. Thus, a person who, based on his education, life experience and world-view, would be classified as a "redneck" in Europe, may be perceived by outsiders as an "upper class professional" based on external status markers (such as income, automobile, or house). In other words, an outsiders may speak to a redneck but think that he is talking to an upper class professional, and then be surprised when his interlocutor shows attitudes that seem inconsistent with his perceived class status.
Wojtek