Unlike many countries, you can be prosecuted for failing a drug test in China, even if not in possession of the said substances I think? If you get arrested for any crime, you may get drug tested. I have never been arrested in my life, so I can't really tell you for definite.

There was the random drug testing raid as Dos Kolegas bar in Beijing, some foreigners apparently got deported, although conflicting reports say that this was a propaganda scare story.

Anyway, my concern is not about your idea, but that tonnes of dodgy characters from Florida and states with similar laws may move to China to support themselves as they are unable to receive welfare payments!