Because there's a difference between consuming something willingly and a shady corporation conspiring with the system to get unsuspecting people hooked onto their drug of choice for personal benefit?
Not if it's legal for the addict (because mental health issue not criminal), but a crime for the pusher/distributor (because you're taking advantage of those w/ addiction tendencies).
Is there? Everything you see on the store shelves is there because some corporation manipulated the store to put it there, and because that corporation expects to personally benefit from lots of people buying it.