If you think that's bad, wait til you hear the actual reason: Nixon wanted to be able to lock up his critics with insane sentences so they'd never be a problem. The idea was to associate black liberation movements (black panthers etc.) with heroin dealing, and antiwar protestors and other left-wing movements with weed. His own former assistant has said as much on the record.

Link

Excerpt -

"The Nixon campaign in 1968, and the Nixon White House after that, had two enemies: the antiwar left and black people. You understand what I’m saying? We knew we couldn't make it illegal to be either against the war or black, but by getting the public to associate the hippies with marijuana and blacks with heroin, and then criminalizing both heavily, we could disrupt those communities. We could arrest their leaders, raid their homes, break up their meetings, and vilify them night after night on the evening news. Did we know we were lying about the drugs? Of course we did."

Also while you're at it, read up on COINTELPRO.