Disclaimer: I am not a lawyer, or even an American.
That said, it
appears to me that Wikipedia is a privately-operated members' club,
and is entitled to specify what is allowed not to be said or done on
its premises, much as the Britney Spears Fan Club is entitled to kick
out people who insist that Christina Aguilera is cooler. Neither is a
violation of US law.
Generally your tastes in pop music are not protected by law. Your
political affiliations may be. Like you, I am neither a lawyer nor an
American, so I don't know what the relevant laws are, however I
wouldn't be surprised if banning Nazis was illegal in some countries.