It does seem obvious, and I'm on your side, but consider the mindset of the other side. There are people, a *lot* of people, who consider using almost any kind of recreational drugs to be deeply immoral.
Their concerns are not born of the violence of the drug business, but the chemical itself. If you miracled up a pot plant from another dimension and used it in perfect isolation, they'd still consider that act to be immoral and worth society's effort to stamp out.
You're not going to win the day with arguments of practicality. The pro-legalization forces have to beat both the folks who think that the War on Drugs is an important ethical position, and the folks who benefit from the growing police state that the War on Drugs sponsors.