My favorite conspiracy theory regarding drug legalization is, "the rich" will want to legalize drugs to get rid of "the poor" on the theory that people who've been put out of work by machines are an unnecessary burden, and that widespread use of drugs among such people is a good way to reduce their numbers.

(I don't quite "believe" this theory in the sense of imagining a bunch of rich people sitting in a room, conspiring about this and cackling evilly. I "believe" in this more in the sense of, I think drugs are very bad for you on average and they certainly are the #1 cause of untimely death among the people I've personally known, and I think that the kind of people making policy decisions will not be that upset if large numbers of poorer people are very adversely affected by legalization, any more than they are upset by the adverse effects of draconian prohibition measures.)