Should / did Nazis in America have the right to wave the Swastika during WWII, when Germany was a sworn enemy of the US? At a time when traitors and spies could be executed for treason or espionage? I don’t know the answer.
That was the last war America declared, so it would be as clear as day. There were German-Americans interned during that war on suspicion of treason, similar to the famous Japanese-Americans (as well as Italian-Americans). I think anyone flying an Italian fascist flag, a flag of the Third Reich, or a Japanese flag would have been subject to more than just a little pressure to take it down.