I’m not sure the question is whether legality completely extinguishes a black market for the goods or services in question. After all, there’s still a black market for individual cigarettes (“loosies”) in poor neighborhoods, even though cigarettes remain legal. It’s a question of whether the situation is substantially and incrementally better in those places where the conduct is lawful than where it isn’t. So if there is less trafficking, I’d say that’s a net win.
