So basically, his argument is this:
Morality is about right and wrong, and that’s what laws put into legal form. Can you think of one law which doesn’t declare one behavior right and its opposite wrong? The truth is all laws legislate morality (even speed limits imply a moral right to life). And everyone in politics — conservatives, libertarians and liberals — is trying to legislate morality. The only question is: “Whose morality should be legislated?”Firstly, just because something tells you what you "ought to do" and the "right thing to do" does not mean that it is a moral statement. It is the right thing to move a rook horizontally or vertically on the chess piece. It is the wrong thing to move the rook diagonally. That is a claim about right and wrong which is not a moral claim.
There are many types of things that are legislated which do not have to do with moral claims. For example, the Australian Constitution gives the power to the commonwealth government to make laws regarding lighthouses (google the lighthouse power). I would like to see Turek draw some sort of linkage between morality and making laws about lighthouses. The Acts Interpretation Act dictates that a court should, when analysing statute, look at the ordinary meaning of the words first. Does this also somehow link to morality? The government could decide tonight to pass a law forcing all people to wear blue coloured shirts, how does this link to morality?
Secondly, let's say that his claim is true, that all legal obligations are moral obligations (which would be a natural law position), it does not mean that all moral obligations are legal obligations. Not everything which we think is a moral rule necessarily should be legislated on. For example a Christian may believe going to church every week is the moral thing to do, but even an extremely conservative Christian would think twice before then stating that we should legislate to force everyone to go to Church. What then is the line? What types of morality should the government legislate on? Perhaps, it should legislate on morals which directly harm people as suggested by Mill, and we see that natural law comes to meet legal positivists halfway.