Yeah, it's complete bull. My guess is it's just a dumb rationalization to make a distinction between making a lethal drug (tobacco) legal while making a comparatively harmless drug (marijuana) illegal.

I wonder how many people have made the leap from not doing any drugs (tobacco and alcohol) to smoking weed. My guess is that the percentage of people that smoked cigarettes and/or drank alcohol before they moved on to weed is staggeringly high.