For a change, I have to agree with you (a scary thought, indeed). The tendency to label a socially unacceptable behavior with an "essentialist" label linking it to a nation, ethnicity, class or kindred seemingly "objective" group is pretty universal. I saw that in Poland where different behaviors were sometimes labeled by "ethnic" labels - for example, crude and boorish behavior was "Russian." Interestingly, this label seldom affected the face to face interaction with people of the said nationality - most people would describe Russians with whom they actually interacted as "warm" and "cordial." An Irish co-worker of mine constantly reminds me how negative stereotypes received the 'Irish' nationality (cf. paddy-wagon) in this country - yet Irishmen are generally considered friendly and fun-loving people.
I think that the explanation of this phenomenon is the behavioral model of transaction cost minimization (i.e. cost and effort avoidance rather than benefit maximization) - people use the hackneyed stereotypes because they are easily available and do not require much thought or explanation, and o not pay much attention to their actual meaning, etymology, or connotations. This is why it is possible to say "n-word behavior," "drunk like an Irishman" "shrewd like a Jew" or "dumb like a Polack" without being a bigot or a racist.
PS. The car radio blasting is not exclusively a "n-word thing" - I see the "white trash" doing that a lot too. I see it mainly as the case of typical "US boorishness and arrogance" - since one does not see much of that behavior outside the US (e.g. in Europe or in Africa).
Wojtek