There seems to be a kind of contradiction in the Rambam. How much of the Law of Moses is natural Law and how much is only because of revelation? In the Guide you see two different things. One is that the laws of the ancient Greeks natural law [as known by Abraham the patriarch] was necessary step towards Matan Torah [giving of the Torah]. So they are different in essence. Yet the Rambam also gives natural reasons for the laws. Maybe he thought the reasons were a necessary, but not sufficient condition?
That is reason that would create conditions for the mitzvot but not the entire cause.