re: Free speech for one is free speech for all. (A restatement of principle -- nothing juicy)
It would seem that another way the launching of personal attacks violates that right of free discussion, in the context of HZ, is that it can force the whole thread to be moved to "Flame," where it will likely get far less traffic. That way, someone who has a personal beef with a particular poster could categorically marginalize that person's comments. In the same way, someone who is uncomfortable with a particular topic could defeat any attempt to discuss it.