I'd be more sympathetic to Mr. Dean if his demands were more reasonable and commensurate to the alleged damage. I doubt Avatar
(and the sequels) is going to hurt sales of his art books. If he had just asked that proper credit be given and some fair monetary compensation, I'd be totally on his side, but $50 million dollars for some artistic inspiration seems excessive, and demanding that the films no longer be distributed or sequels be made unless they completely remove his influence from the designs just seems needlessly vindictive.
While I don't think Cameron has any malicious intent when he borrows these ideas --because we're all inspired by past works when we come up with any new stories or art-- he really needs to learn to shut the hell up when people ask him direct questions about where the idea came from, because he's his own worst enemy.