I would say, rather than Molyneux simply looking at things the wrong way, that his system doesn't in fact work at all. Sure, it may work in most situations, but consider the following:
It is not preferable for slave-owners to free their slaves. It would be preferable for the slaves, but not for the slave owners, because they would have no advantage in freeing the slaves. The question that morality is supposed to answer is: what motive do the slave owners have for releasing the slaves?
In Divine Command Theory: "Thus saith the Lord," and you better listen to him!
In UPB: None.
The point is that a moral source must care about every being in order to be compatible with what our consciences call morality.
In UPB, the moral source, "that which is preferable," is not the same for every individual, and allows people to take advantage if there are no bad (earthly) consequences.
Morality is a question of consequences or judgement. If not, then a person might say, "Why should I bother being moral?" The reason Divine Command Theory alone works is that it is the only moral theory in which all sinful acts are judged. All naturalistic moral theories leave many times when that which our consciences say is immoral does not have negative consequences for the perpetrator—and are, therefore, in these theories, not within the bounds of what characterizes that which is "immoral."
I ought to write an article on the Moral Argument soon.