Are things just because the law says so, or does the law declare things just because they are just? If the first, then justice is whatever legislators say it is, which makes justice a creature of whimsy. If justice is a creature of whimsy, then the objection to jury nullification on the grounds that it is whimsical is no objection at all. If the second, then justice exists independently of the law, and law’s purpose is simply to acknowledge it, if the law fails to do so, then jury nullification must step in to do so.
The Euthyphro dilemma, which Atheists love to hurl at us Theists, can also be applied to something as thoroughly secular as legal positivism—with the same devastating effect.