I don't---studies like the one I cited mistake correlation for causation. Occam's Razor discards the "gateway theory" in favor of the explanation that earlier use of one drug and later use of another have a common cause in a predisposition to alter one's mental state.
I cited that passage on "gateway" alcohol and tobacco to point out to presidio9 and other WODdies that if they argue for retaining the marijuana ban because it's a "gateway drug" then they must also support the banning of alcohol and tobacco on the same grounds.