Because we live in a world of constraints, prudence tells us that if we cannot prohibit a social evil entirely, we can limit it through appropriate fences.  Building fences around a social evil, as part of a larger strategy to secure justice, precludes what can be prohibited now without admitting the legitimacy of what remains unprohibited.  By limiting the harm done or lessening the negative consequences, we do not admit or support the rest of the evil that we do not have the power (legal or political) to touch now.