The clue may be in the word "history"...
As in, in the past?
The warrant may have found nothing and may have been based on faulty information, you are maybe aware that you get a search warrant to find evidence and there is often a very good chance you won't find any evidence for the crime you suspect has been done.
You can reform from being a criminal or a drug user, and given in the US people are severely criminalised who may only have ever done small amounts of drugs because apparently having a user amount of drugs on you is a terrible crime worse than many violent ones thanks to it being an easy win for police, prosecutors and politicians (although if you've got access to money and a decent lawyer you could often get it thrown out before being officially charged).
Also charges do not equal convictions and a misdemeanour conviction could be he got caught with some weed and convicted...(a misdemeanor is a low level crime, so either possession of a user amount, or possibly a little more of weed).
Some people seem to think that no matter what any criminal conviction means you're always going to be an active criminal and it justifies anything that happens to that person henceforth.