Moore made a lot of bucks off of others' creations, so I don't know what he has to complain about.
I think the main difference here is that Moore thought he had a contract with DC that would give him ownership of Watchmen after a few years. It turned out that he hadn't read the fine print very well, and he was wrong. So he still resents DC for not being willing to honor what he thought was the spirit of their agreement, rather than the letter of it.
Moore doesn't feel that he has wronged anyone in quite that way by his use of other people's characters. We can argue over the finer points of various cases, but overall I think he's mostly right.
It would probably be good for Moore if he could let go of his old grudges, I guess. But they're his grudges, he gets to decide how to handle them.