"For instance, you can post security guards and have rules against assault."
Because as this thread shows, people always follow the rules and don't complain about them. It is already against the rules to assault anyone (not to mention the law), and security guards are only helpful once they arrive. That 45 seconds it takes them to knife through the crowd to get to the altercation could be very dangerous.
"Presumably, the state dinner example you gave would occur in a "non-public forum," where speech restrictions can be imposed so long as they are "content-neutral and reasonable in light of the forum." Requiring formal dress at a fancy dinner meets this requirement."
I'm not so confident that the STUDENT SECTION at a university basketball game is really a public venue. It was intended for a specific audience, and tickets were only sold by the university TO that audience. Nobody else. If those are then transferred third party, I don't think it is the university's fault if that third party is unhappy with some of the restrictions placed on them at said ticketed event.