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.
A lawyer is someone who practises law, or perhaps studies it, an attorney (in the US) or a barrister or solicitor (in the UK), someone who might have little or no interest in the philosophy of law, or in the the concept of law in the abstract. A legal philosopher, on the other hand, is a philosopher, often or typically today an academic person, one who studies and perhaps contributes to the philosophy of law. The philosophy of law is one of the subject areas within philosophy (such as the philosophy of science, the philosophy of religion, ethics, the theory of knowledge, metaphysics, and so on) in which one studies the question 'What is law?' - to be distinguished of course from the question 'What is the law?' - which is not a philosophical question at all. Or a philosopher of the law might be interested in the question what it is that makes law "valid". There is little doubt what the laws were in say 1935 Germany, e.g. the Nuremberg laws prohibiting the marriage of citizens ("Staatsangehoerigen") and those of Jewish descent, and declaring even such marriages conducted outside Germany invalid within Germany, or the law authorizing among others the sterilization of those with bipolar disease; but were the laws themselves valid, resting as they did on an "Enabling Act", passed by the German Parliament, that abolished the Parliament and gave to Adolf Hitler absolute power? If this act was not valid, why not? Are there limits, perhaps based on "natural rights", as to which laws can be validly enacted? A very important and sane book written by someone who was a distinguished philosopher of law and had also been a practising lawyer is H.L.A. Hart's The Concept of Law, (Oxford: OUP, 1961). Jurisprudence is a wider field than the philosophy of law, though it includes the philosophy of law, containing in addition more narrowly social and political aspects of the law. None of this really has to do with the distinction between metaethics and normative ethics. The legal philosopher might be profoundly interested in normative ethics, as for example H.L.A. Hart was. His influential contribution to the discussion leading up to the decriminalization of homosexual acts in the UK in 1967 was a sophisticated version of the position that the only moral limit of the criminal law is harm.