“How does the poor schlub at Western Union know know his customer is legal?”
Wouldn’t matter. The tax would be on the destination of the wire transfer. Wouldn’t matter if it was you sending money to bail your daughter out of a jam.
I'm assuming that this is based on some claimed national security threat else the FISA court would laugh.
So what happens when Mexico says "OK, here's the money?" Do we let the remittances resume, thus making it clear that the claimed threat was a sham? If not, what's Mexico's incentive to negotiate?