<Isn't the right to an opinion one of the most fundemental freedoms? Of course calling a divergent opinion "anti-American" might just be intentional; sort of a way to prevent someone from expressing an opinion.

There are opinions, and then there are opinions. A university professor who stood in front of a classroom and gave the opinion that black people ought to be slaves, would be pilloried, and should be. Okay, the government wouldn't arrest him for his opinion (not yet, thank goodness), but damn sure he would lose his job and probably never get another one.

I am just puzzled as to why advocating the mass slavery of Marxism is not only tolerated on campuses, but actively encouraged.

"Anti-American" has indeed been used improperly to suppress opinions. So, for that matter, has "communist." Other words used in such suppression, in more recent times, include "racist," "homophobe," "fascist," and many others.

The use of these words by unskilled debaters to simply try and shut up an opponent does not mean that they cannot be used accurately in the right context. There are real anti-Americans, just like there are real racists.