The viewpoint neutrality principle prevents public universities from discriminating against student organizations on the basis of the views they express. For example, they can't sponsor a "gay rights" student group while banning a Christian group for the sole reason of the organization's viewpoints. Students are permitted to withhold activity fees when such discrimination exists.
In the area of university sponsored speech, the Court has decided two vitally important cases, in 1995 and in 2000, which both clearly held that universities must remain viewpoint neutral when funding student organizations. Viewpoint neutrality means that public universities, in making their decisions about funding, may not take into consideration what position or opinion a student or group of students stands for or advocates. Free Speech - The Basics - FIRE