Consider this: Marijuana was originally made illegal not based on any sort of scientific study, but rather on racism towards blacks and Mexicans.
It definitely entered into the equation, but I think economic incentive, i.e., eliminate the competition, was the primary goal (for Hearst, Dupont, et al). Racism and self-interest on the part of the Federal Bureau of Narcotics, or what ever it was called at the time, were piled on to clinch the matter.
As things stand now, I think economic incentive
still plays a huge part of this. Think of how many drug warriors would be out of business were the laws changed anytime soon. And this is really just looking at the bottom echelon. The real movers and shakers in all of this operate far above the fray of ideologues they employ to do the dirty work.