My understanding (from paying a small amount of attention on twitter, not serious research, so grain of salt) is that part of the problem is that "simple marijuana possession" is almost _never_ levied as a solo crime. Even for first time offenders, it comes paired with "intent to distribute" or "resisting arrest" or "was rude to the cops who asked to search your car" or "let's add bonus charges to put the black person in jail for longer".

It is a good first step though, and I hope we see more of them!

~Sor