Harsh anti-drug laws seem irrational only if viewed as a response to pharmaceutical properties of drugs. In reality the legislation attacks drug users for daring to question middle class values. That is the real crime. To argue for drug law reform on the basis of drug properties is irrelevant unless that argument is used to draw out citizens' underlying fears. Those must be addressed.
Scapegoats are useful politically.
They are a shorthand way of uniting heterogeneous citizens into a common group identity, a cohesion that depends on opposition to the Evil Other. If the Evil Other were to disappear, politicians would be harder pressed to establish their own goodness. Whether drug control laws actually control drugs is irrelevant to the statutes' purpose. Otherwise they would have been reformed long ago. The point is to punish scapegoats.