As far as I understand it, about 4 years ago, one of my brothers lost his visa and had a hell of a struggle to get back to the US to finish a couple of affairs like sell his RV he had bought (he'd pretty near sold up everything he had to buy the RV and have a planned year in America) because a "friend" of his was caught growing a cannabis plant in his house (in the UK) and blamed my brother who pleaded guilty and then had a criminal record...

Apparently the US certainly had systems and enforced them then... Scary to think what they made a fuss of and what they let through...

Jon