Ah, I had never seriously associated the expression with that. I'd heard of the committee, and have some shallow understanding of the whole McCarthy era (as we vaguely call it over here), but I have always dismissed that - as far as idiom/the vernacular language goes - as a corruption and a glitch.

To me (not an American), unAmerican still implies "a bad sport", it doesn't carry the negatives associated with the McCarthy thing - which, to me and my peers, was decidedly unAmerican - a perversion of the sentiment that's for decades been acknowledged as evil and dismissed accordingly.