My hedshop is pastede on yay

Can anyone give me a serious, hard-evidence-supported, logical argument for why pot shouldn't be legal -- even if you yourself don't agree with it? I already know the history of the criminalization of marijuana, and the politics behind "the drug war"; that's not what I'm looking for. I have looked around at various arguments on Teh Intartruck (yes, I know, that in and of itself means the many of these "arguments" are likely not the most cogent or level-headed), and I have yet to find a one that makes real sense to me. Some of the arguments out there are so riddled with logical fallacies that simply reading them brings on some of the effects of getting stoned--dizziness, time dilation, confusion, giggles, nausea--though you'll have to come up with the munchies on your own.

This is not meant as a facetious or cynical question; I seriously want to know if there is a rational position why smoking (or buying or selling) marijuana should be illegal which I could understand.

Disclosure: I don't smoke pot, but I cannot think of a single reason it shouldn't be legal. Hence the post.