Not only should it be legal, it should be highly publicized and ridiculed. Someone once asked Jerry Springer why he, a Jew, would give KKK members a platform to spread their hate to a national audience. His response was sublime. He said that it is our obligation as a free society to allow anyone to speak their hate, but it is our duty to make sure everyone recognizes it for what it is. If we don’t refute something because we’re too busy burying it, then the “truth” becomes whatever they want it to be.
