[LouisSayers]

Yes they do, if they have reason to believe you will break US law (I.e. smoking weed) they will not hesitate to cancel your visa and put you straight back on the plane.

They've shown that multiple times in their border control TV show.

The episodes I saw:

There was an Australian girl (admitted to smoking weed on a previous trip) - put straight back on the plane, visa cancelled, and a German guy- on his way out got found with a speck of weed, admitted to smoking weed - got his visa cancelled and told he'd be refused entry if he tried to return.

They basically say "we are federal officers and we enforce federal law". In terms of visas if you provide reason to believe you will break the law, then they have the right to refuse you entry.