You're forgetting black markets. Drug dealers and gun runners and human traffickers are already breaking serious laws. They don't care about breaking other laws. If they get caught they're already going to jail whether they use illegal math or not.
The lack of utility also assumes that all countries would ban it, which seems unlikely, if only because some countries have the incentive to do anything they can to stick it to other countries they don't like. Then it still retains value globally because it can be turned into real stuff in those countries.
The real issue is this: You can't ban it for the "bad stuff" that gets used as the excuse to ban it, because those guys will just break the law, as they already do. You can ban it for innocent people using it for innocent things, but that's just stupid.
Or corrupt, because incumbents don't like the competition, which is the strongest driving force for such laws.