I agree with you, but the hacker is still a thief. In the same way that the stupidity of a greedy individual losing $50,000 to scammer claiming to have 50 tankers of oil in the Atlantic does not magically turn the thief scammer into not-a-thief-or-scammer.
WHO invented the funny rules ? It wasn't the hacker, it were the DAO guys ! Those are equivalent to the scammer telling you that he has 50 tankers in the Atlantic. It are the DAO guys who told you (in their description) that DAO funds were only to be moved by a vote, and *at the same time* told you that only the code is the law (and that code didn't only allow for funds to be moved by votes, but also by tricky tricks the hacker found out).
So who was the lying scammer here ? The hacker who saw the trick, or the DAO guys making people believe things that weren't true ?