There are certain restrictions on illicit drugs too, but they don't work very well in practice. Everyone involved in such transactions has every incentive to do

There are certain restrictions on illicit drugs too, but they don't work very well in practice. Everyone involved in such transactions has every incentive to do what they wanna do anyway. What this landlord is doing is the equivalent of saying, "People are going to do drugs. OK. You can sell drugs and you can consume drugs, I'm not going to stop you. I only demand to test their purity first. And if you hurt unrelated parties I'm going to hold the guilty party accountable." Now everyone has something to gain by playing by the rules. Sure, they have to give up something, but they get more than they give if they're honest. Once someone in a legal position of authority can get the parties to agree to play by the rules, that authority can do a lot to reduce, if not completely mitigate, the negative side effects.