“To be sure, one value comes with every positive-law statute without reference to its content: Any statute is always better than no statute at all, since it at least creates legal certainty. But legal certainty is not the only value that law must effectuate, nor is it the decisive value. Alongside legal certainty, there are two other values: purposiveness and justice. In ranking these values, we assign to last place the purposiveness of the law in serving the public benefit.”