I think I have just come up with a conspiracy theory that the real conspiracy theorists have not thought of yet. See how you like this:
Legalizing marijuana is a plot to weed out the weak, and thus strengthen the human species. Making it legal removes the largest barrier people have to getting it, so they will buy it more often, so they will smoke weed and do nothing with their lives, thus weeding themselves out.
(Actually I think legalization of marijuana is just giving the voting public what they want, but it's more fun to make it a conspiracy theory.)