The same thing that induces universities to behave reasonably well as neutral platforms. It is a combination of pressure from public esteem and the moral principles of the university (both the principles of individual administrators/professors and the principles explicitly endorsed by the organization as a whole). And all of that is interlinked.
It is an awful misconceptions that the only way to effectively get other people to act differently is with law. It is lazy and leads to unnecessary use of force. Most things that are bad are not illegal (e.g., lying to your friends, being mean, abandoning your parents, cheating on your spouse) and yet occur at much lower rates than they would without the many extra-legal mechanisms that keep them in check.