They aren't discriminating against whites. It's just that users are easier to catch. And when they get out, they start using again, and like as not get arrested again. The notion of combing prison with rehabilitation is a bad one. It sounds good to middle-class white voters, which is why rehab exists.
People like to feel that they're doing good.
The truth is that if you put a guy in the slam for using, or for stealing to buy, the first thing he's going to do when he gets out is to look for drugs. Rehab is like vocational training to teach criminals to be computer programmers. It seldom works. The rehabbed user goes back to the same streets. He's quickly back into the life.