Basic argument is that a law-abiding student will suddenly go nuts over a class discussion and a grade. Bang, bang, bang - I disagree with your view of the relationship of quantum mechanics and general relativity.
No consideration of the non-law-abiding rampager killer showing up (who wouldn't care about the law) or defended yourself against crime.
Faculty would be scared to offer an opinion. Well, if you are a wussy old prof, why can't the disagreeing student just run up and give you a beat down?