But they are natural, aren't they? No less natural than the watermelon we are all eating (it might be cultivated), and I don't see anyone wanting to criminalize eating watermelon. Weed, tobacco, alcohol, coca leaves, poppy, they are all natural.
Also, even if it's natural, it has nothing to do with its dangerousness, so I don't follow your argument.
With regards to legalizing it, I'm on the side that if alcohol and tobacco are legal, weed should be legal, too. They are all bad for you in some sense, but we need to balance that with freedom and individual choice, and to me, they are pretty similar.