They likely won't catch every bit of AI generated code, but I don't think that's the point. In my view the point is to take a stand and try. It also signals that the authors care about their code in a certain way.

There are all sorts of things that are impossible to perfectly enforce, but that doesn't mean we just give up. Insider trading is extremely difficult to prove, and I bet more people get away with it than don't, but I still think making it illegal is a good policy.