So morality has a lot to do with logic — indeed I have argued that moral reasoning is a type of applied logical reasoning — but it is not logic “all the way down,” it is anchored by certain contingent facts about humanity, bonoboness and so forth.
But, despite Yudkowsky’s confident claim, morality isn’t a matter of logic “all the way down,” because it has to start with some axioms, some brute facts about the type of organisms that engage in moral reasoning to begin with. Those facts don’t come from physics (though, like everything else, they better be compatible with all the laws of physics), they come from biology. A reasonable theory of ethics, then, can emerge only from a combination of biology (by which I mean not just evolutionary biology, but also cultural evolution) and logic.
http://rationallyspeaking.blogspot.de/2013/01/lesswrong-on-morality-and-logic.html
When we talk about morality, we are talking about those contingent facts, and once we've pinned down precisely what the consequences of those contingent facts are, we have picked out a logical object. We are not trying to explain why we picked this logical object and not some other logical object - that is anchored by contingent facts about humanity, evolutionary biology, etc. We are just trying to describe this logical object.
This point might be made more clearly by Sorting Pebbles Into Correct Heaps. Why the pebblesorting people choose to sort pebbles one way and not another way is anchored by contingent facts about pebblesorting people, evolutionary biology, etc. But the algorithm that decides how the pebblesorting people sort pebbles is a logical object.
It doesn't matter where our morality comes from (except insofar as this helps us figure out what it is); wherever it came from, it's still the same morality.
Mainstream philosophy translation: moral concepts rigidly designate certain natural properties. However, precisely which properties these are was originally fixed by certain contingent facts about the world we live in and human history.
Hence the whole "If the world had been different, then what is denoted by "morality" would have been different, but those actions would still be immoral (given what "morality" actually denotes)" thing.
This position is sometimes referred to as "sythetic ethical naturalism".