People keep saying, "free speech!", but our right to free speech is something the government isn't allowed to restrict. It doesn't apply to a privately owned website... just as the host of a private dinner party may choose to limit the discussion at his table. If you don't like it, eat elsewhere.
I'm a moderator on another aviation forum. We don't allow political discussion, because not only does it get ugly, it dilutes the legitimate discussion and makes people stay away who would otherwise have something useful to contribute.