I am a Brit, but respect the usage of "unAmerican". To me, the equivalents on this side of the pond are expressions such as, "That's just not cricket", or, "Do have a care", or "Steady on, now."
These are idioms that express a culture, they are not intended to define rigidly some rules, but rather express a sentiment about shared and almost undefinable values.
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.
These are idioms that express a culture, they are not intended to define rigidly some rules, but rather express a sentiment about shared and almost undefinable values.