If you read my posts you'll see that I acknowledged that shouting someone down isn't illegal, but reflects poorly on the students and if you think that any person has an obligation to prevent someone they disagree with from speaking then that effectively defeats the purpose of not allowing the government to interfere with free speech.
Except that our whole constitution and legal system are premised on the difference between public and private actions and actors. My mom can punish me for taking the Lord's name in vain; the police can't.
I'm not saying what they did should be illegal. I am saying that it shows they don't care about free speech. It is true that if everytime someone got outraged they behaved like this we would have much less of an open public dialogue.