Well, if the rules are the same as an actual public square, then outgroups can participate in exactly the same way as they do in actual public spaces, governed by laws enshrining freedom of expression. That is to say they may be insulted and ridiculed by members of the public because we as a society have decided that we don't trust any authority to moderate public discourse.

These conditions do in fact represent "actual" freedom. Basically if you support people insulting and ridiculing Nazis (which I do), you should support them insulting and ridiculing anyone else on the same principle.