This movie about cannabis prohibition is worth watching. It rehearses all the major arguments for legalizing "the safest drug on the planet". Along the way, it brings up the problem of private prisons in the U.S. That's a really good point. If it is possible to make money by running a prison, then there's a natural (in the sense of economically rational) basis for a lobby for criminalizing all sorts of activities and, of course, for tough sentencing of offenders.
There is a of course a similar problem with another human activity: war. A great deal of policy today is being shaped by lobbies who have direct economic interest in crime and war. For some, it is economically rational to start wars and criminalize activities that many people like to engage in. The interest is there regardless of any strategic advantage or disadvantage that a war might bring about or intrinsic harm that the activity in question may or may not cause. There are people who have their own reasons to warmonger and to criminalize.
I'm not making any accusations. I'm just noting, objectively, that those interests exist. The solution is quite simple: put corrections and munitions in the hands of the state. Accept the inefficiency that this might involve in order to avoid constructing this kind of interest.
PS: This argument can also be made in the case of medicine and finance, by the way. And countless other things no doubt. There are certain problems that a society should not let its members profit from solving, because, if they do, members of that society will have an interest in producing the problems themselves.