Oh, I know why it's bad.

The year after a device that would make hemp more easily processed for use in clothing and other such goods (think cotton gin), marijuana is declared illegal. And who was smoking this "demon weed"? It came from the spicks, and smoked by the blacks. Therefore it MUST be illegal.

By the way, I wasn't trying to be racist, I was calling the people who criminalized marijuana racist, which they were.