Reductio ad absurdum is not a very good argument here, IMHO. As usual, in real life a sense of proportion has to be applied. It is normal and acceptable for societies to outlaw behaviors that entail significant negative externalities, or in the case of minors, prevent them from doing something that harms only them. The question is, however, to what extent drug use, and of what drugs exactly, is such an activity that should be outlawed.

That question is best answered by medical research and weighing those social ills that stem from drug use itself against those that are the result of criminalization thereof.