"Un-american" is one of the scariest, most despicable words in the American language (brits are thankfully exempt of this word).
Only people entirely oblivious to world political history can afford to ignore the putrid odor of fascism exhaled by the usage of this word (from the 50's House Un-American Activities Committee proceedings to our days): at its best, it justifies censorship (thus, blissfully ignoring 1st Amendment Rights), at its worth it is used as an excuse to legitimate inquisitorial proceeds against any kind of political opposition.
"Un-american" belongs to a solid line of thought, that seems to consider that in order to preserve democracy (or some people's idea of democracy, as the case may be), one can and must go to the greatest lengths to gag dissent.
The mere word "un-american" is the most un-american concept there can be...
"Un-american" belongs in the same word category as "political purge", "gulag" or "freedom control" and this is not the vocabulary of a free democratic country.
And if you believe totalitarianism needs concentration camps or summary execution to be frighteningly effective, then you need to read Orwell and Huxley again...