The issue is that once the catalytic material is removed from the converter it is basically impossible to prove it was not stolen. So you’re right you cannot enforce it that way. So create requirements that catalytic material sales needs permits and licenses. Or that one-off sales need to provide proof of purchase and that the place purchasing the catalytic converter must obtain prior proof of purchase and retain that proof for a period of so many years.

There are ways to do this, we just aren’t.