"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?"

This wouldn't actually be the case under his system. The nomenclature is important. Universally preferable, not simply preferred. Basically, what would be the most consistent with itself, and for all parties involved. The problem, as has been noted, is that it lacks an authoritative support.