You are vastly overestimating the federal government’s coherence and coördination. Yes, we use black hats. Yes, we still jail and sanction them.
The first thing you do after conquering the throne is to bundle up all your pending atrocities in one and eliminate competition. The second thing you do is slaughter the mercenaries you had hired to win your war of ascension.
No reason to leave them around and let the next usurper hire them to dethrone you.
Also, there have been some enforcement actions on intermediaries for facilitating such payments - e.g. https://home.treasury.gov/news/press-releases/jy0471
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.
No, that’s what the sanctions threat is for. It may be possible. But now you’re in the company of money launderers and terrorism financiers.
To be clear, I don’t think this is necessary. But it’s naïve to imagine it’s beyond D.C.’s capacity.