What would serve as a good excuse for doing immoral things, or not knowing right from wrong?
The lack of anything depending on whether an act was immoral; the lack of any consequences to not knowing right from wrong.
Firstly, you are assuming something that many would disagree with: that an act with no consequences can be immoral, rather than being automatically morally neutral.
Secondly: even if true, that is a special case.
The importance of morality flows from its obligatoriness.
There seems to be a widespread impression that the metaethics sequence was not very successful as an explanation of Eliezer Yudkowsky's views. It even says so on the wiki. And frankly, I'm puzzled by this... hence the "apparently" in this post's title. When I read the metaethics sequence, it seemed to make perfect sense to me. I can think of a couple things that may have made me different from the average OB/LW reader in this regard: