Kip Werking says: "[T]here seems to be no principled reason for regarding one [type of moral precept], and not the other, as non-arbitrary. In both cases, the moral content is discovered, and not chosen, one just happens to be discovered in our DNA, and not in a tablet."

Though there's a question whether moral dispositions exist encoded in our DNA that can ground some properly moral norm or set of norms, such dispositions would be far less arbitrary than a norm inscribed on a tablet. These dispositions might be "arbitrary" in the sense that evolution might have gone differently. But given it went the way it went, our genetic dispositions have a de facto, if not de moralitas (hope that Latin's right), claim on us that a tablet doesn't: I can't abjure my own operating system.