Yet they still complied with U.S. sanctions. (Or were arrested abroad for defying them.)

You seem to misunderstand that sanctions are not a treaty obligation. If your country deals with a sanctioned entity, it gets sanctioned as well. That enforces compliance indirectly. America and and does unilaterally extend sanctions.

We don’t sanction money launderers generally. And no, terrorism finance isn’t a solved problem either. Hence why I said one would need to keep company with that category of people were such a measure enacted. But again, your K&R retiree cum schoolteacher was describing a political constraint. Not a functional one.