Eliminating the black market is good. Having huge corporations pushing drugs is maybe not so good, or at least we should think through the consequences.
I've read that this approach has been pretty successful where it's been tried. the theory is that doing it in a controlled environment removes a lot of the "fun" for the users so more of them eventually quit. additionally, it makes the use of hard drugs less visible, so fewer people end up picking up the habit.
I have to suspect that there's a point where you introduce too much friction though. if it's too annoying to go to the government facility, it might become tempting to just buy illegally. for instance, there is still a market for bootleg cigarettes even though anyone 18/21 or older can buy them at any gas station or convenience store. for some people it's worth buying them illegally to either evade the tax or purchase less than a full pack at once. I've also heard from my friends in CA that there is still a thriving illicit weed market for similar reasons.
Lots of vested interests in continued criminalization of life, liberty, and the pursuit of happiness.
I swear if we forbade private incarceration, we'd see a major change in both total prison population, and the sort of "crimes" people are shanghaied for.