Why is it allowed? Probably has something to do with the Constitution.
I'm glad these people are coming out, dressing like communists (black political graphic t-shirt over white long sleeved t-shirt seems to be the new communist uniform) and showing all those interested that anti-Zionism is anti-Antisemitism.
The only tragedy here is that "respectable" Jewish groups like the ADL and Simon Wiesenthal Center don't focus the full magnitude of their oppressive powers against the "anti-Zionist" left, the greatest threat to Jews. They're still busy trying to bring Nazis to justice, even though all of the officers have long since died and Nazism has been reduced to nothing more than the butt of a joke.