Religion is a poor source of morality

[Mudhammam]

I think Sam Harris' primary point is that whilst theists argue for objective morality that's ontologically objective, he argues instead that subjective facts about the well being of conscious creatures can be studied objectively in an  epistemic way instead... the same way science studies other things epistemecally . And his point is that the conclusions matter to the well being of everyone.

To paraphrase Sam Harris: "If words like "good" and "bad" and "right" and "wrong" mean anything at all then... they mean we should at the very least steer away from the worst possible misery for everyone."

He's not saying that it can be proven ontologically, he's saying that you can epistemically study subjective facts about well being that matter in an objective scientific way.

Although I enjoyed the Moral Landscape very much, I found it somewhat lacking for the very reason that he doesn't offer much in the way of a defense for the ontological status of objective moral values. To me, it makes little sense to claim that there are objective facts to be known about the differences between good and evil if you do not first acknowledge that good and evil are states which exist independent of one's subjective appraisal of those differences, much like the necessary truth that 2+2=4 regardless if a person has learned arithmetic.