Consider two otherwise identical worlds, in one of which determinism is true, and in the other of which it is not. In the second world there is freedom of the will. What difference does this make to the legal system and the moral system? Ex hypothesi, none. When I wrote that if hard determinism is true, moral systems cannot be corrupt, I was describing Stephen's view, not my own! Hence I think the apparent contradiction that he detects. Here's another way of putting the point I wanted to make. We can imagine a deterministic world in which people are thinking good thoughts and doing good things, and making good laws. The world of law and morality seems to have survived the loss of freedom! In this world, some philosophers believe in determinism, and some in freewill. But of course the having of those beliefs is part of a deterministically produced system. There could be incidentally be a deterministic world that is morally and legally better than a world with freewill. Consider the worst world imaginable, but having freewill, and the best world imaginable, but lacking freewill. You might prefer to live in the unpleasant world, but that doesn't mean it's better. In it the innocent are tortured, unfairness abounds, and so on.
If we have no free will, then is the entire legal system redundant since no one can be held accountable for anything since no one has control over their own actions?