Look, you prefaced your comment with a question that has a simple answer. You don't like that answer because it doesn't fit your premise that the law is never helpful to anyone. But it is the answer.

There is plenty of room to discuss the effectiveness/costs of making drugs illegal and how (potential) enforcement should be handled. To whit, your response is answering an entirely different question than the one you originally posed!

EDIT Response to below. I can't understand your position. How is the data we do have even remotely relevant? Its a study about enforcement and so presupposes a population that is excluded by the very question posed...