Cell growth? Yeah, through cancerous tumors. I don't think that's the growth I want.
As for why other "worse" drugs are legal, note how they tried to criminalize alcohol. It was prohibition, and remains the only clause ever put into the constitution restricting people's rights. It's also the only clause to ever be repealed, because it didn't work. That's why marijuana isn't legal- once you do so, there's no turning back. There've been studies that claim both directions, yet I have to say that the more credible ones are the ones that show the negative effects. Potheads only look at the ones that suit their wants.