The professor surely knows that cheating is near universal because the incentive structure of modern life makes cheating optimal in expectation. Nothing he can do will change this fundamental fact. So why bother?
No, I didn't mean that I cheat, and therefore everyone else cheats. I meant that the one time he got to see what's going on, he found that nearly everyone cheats. Unless he has some evidence to the contrary, a reasonable conclusion is that cheating is near-universal.
Evidence to the contrary could be, for example, that this class was very different from a typical class in some way, etc.