Why not? At what point is it OK to let people exclude each other? We don't all have to be friends and play together, do we? We're discriminatory, hierarchical, social animals. Some people will always be excluded. Using authority to try to solve this problem is futile and draconian.
> And private fora are not town squares
I agree, but the question here is whether it's worthwhile for this particular private forum to operate under the same principles of free speech. Certainly some people seem to think so.