Is Mr. Peterson an Atheist? I wonder if he'd stop class if an Agnostic, registered or unregistered, was present. Oh that may be taking it too far... okay, what if he's morally opposed to tattoos? Stop the class.
A knife can be just as deadly. Would he stop the class then? No? So he's not worried about the safety of his students, he's worried about pushing his own agenda. Of course I cannot prove that he has no issue with knives in class, I'm just musing that the former would be the case.