It was on this day in 1933 that Prohibition ended.
Shortly afterward, marijuana was criminalized, in no small part because the alcohol lobby has always been more powerful, and in the 1930s popularly associated with a different ethnic group, than the marijuana proponents.
I was going to provide links to scholarship to support this point, but there isn't a lot of it out on the Web right now. Even the relatively de-politicized National Institutes of Health and the Journal of the AMA have a dearth of information about the relative dangers of pot vs. booze. (And yes, despite the flap about Plan B, I think the NIH are relatively apolitical. The FDA, on the other hand, not so much under this administration.)
I did find a link to a site about Reefer Madness, which is hi-larious. But it's not really scholarship.
Note: I'll stop parroting the History Channel's daily list for now. But if you're interested in histroy, you should subscribe to it.