This is not the only place in criminal law where we have asymmetrical criminal law. We have asymmetrical criminal law in suicide. It is not an offence to commit suicide; it's an offence to aid someone. That's asymmetrical.
It is an offence to sell drugs; it is not an offence to purchase drugs. There is no offence of buying drugs. It is an offence to sell drugs. However, the buyer could of course be criminalized by aiding the seller to sell the drugs, by—