Its actually an actually of having a healthy perspective on your obligations and others rights. Taking upon yourself the impossible obligation of knowing all possible ends puts you in a position to put upon them an unreasonable demand insofar as insisting that someone tell you how they plan to spend that buck.

Even if they are going to spend it on drugs they wont tell you so you are forcing them to degrade themselves either way by lying or by proving they aren't going to spend it on drugs.

Just say no if you think they are going to blow it on drugs or say yes without pretending their drug habit is the result of you giving a buck on a particular corner.