I agree with what you wrote; but I don't think you singled out what's going wrong in The Moral Landscape.

Sam Harris has an argument against absolute moral relativism. There really are absolute moral relativists out there, who say that any moral code is as good as any other, and no one should think poorly of Jeffrey Dahmer because he likes to murder men and screw their corpses. I think there are even people reading LW who think they think that. And Sam says, That kind of talk should not be admitted into the discussion. If you can't pass the bar of saying "hurting people is bad", then you shouldn't be allowed to help work out the social contract. We should all be able to agree that hurting people is bad.

Furthermore, the people who are hurting other people badly and systematically, really do believe that hurting people is bad; they just have demonstrably false beliefs, religious or political, that cause them to think that their actions are helping people in the long run. This is largely true, though I don't think Sam understands the mentality of believers as much as he thinks he does.

So, much pain and suffering could be prevented if we said, Hey, you say we need to do X because Y, so let's figure out whether Y is true... with SCIENCE!

The problem is, this is not enough material to write a book. So Harris makes his claim much more sweeping than it is. He throws people who say that different societies can have different values in together with absolute relativists, to make it seem like Harris is a lone voice crying in the wilderness. He argues, erroneously, that his one simple principle is enough to build a moral code upon. When I got to the part where Harris takes the correct objection that minimizing total harm may be unjust - and instead of agreeing, argues that minimizing total harm will happily work out to be perfectly just because deep inside everyone is unselfish and wants other people to be happy - I gave up and stopped reading.