Billions of dollars are spent prosecuting and jailing cannabis users, growers and merchants. The money comes from the taxpayers and goes to the companies and government agencies that do it. If you draw a surface that envelops the prison and policing industry but excludes taxpayers, it will look like billions are flowing in from weed enforcement.

Private prison industry. Incentive wise they have every incentive to fight cannabis reform. Easy convictions -> more convictions -> more demand for prisons.

google "war on drugs" and "prison industrial complex"