They can't do everything with it. They do anyway, because they're too big to lose a case like that. He cares, that's why he did this.

How it damages DC? Well, imagine the same happened to Superman. Everyone can now legally sell Superman merch. That means you have to go from a 1000% markup to some "measly" 50% if you want to compete at all. And everyone can create new comics and sell them legally. There's nothing preventing you from advertising it as "the new Superman comic you've all been waiting for".

Now the funnier part: even though Marvel is generally more loved and known among people who aren't comics fans, 2 of the 3 most well-known heroes (Superman, Batman, Spider-Man) are DC. Now imagine MCU being able to legally include Superman, DC would be pissed.

Another thing is that if someone wants to damage the IP, they can now. Legally selling a comics where Superman decides to kill all black people? Perfectly legal. How will people like Superman afterwards, I wonder?

Sure, Fables isn't Superman and is nowhere near in popularity - I used it as an example because it's easier to imagine - but it's still popular enough that many people know about it (partially thanks to the The Wolf Among Us game) and it could/will hurt DC in the ways I mentioned.