Conspiracy theories are clever, however this one as a single major flaw.

The legalization of marijuana is a decision that's VOTED ON BY THE PEOPLE!

So it's not the government that's deciding to legalize the drug, it's the community. So unless the community is trying to dumb themselves for the purpose of suppressing mutiny, which is highly unlikely, the reason marijuana is becoming legal is because people want to be able to smoke weed without criminal consequence.