why was this released by warner

It's following TV series aired on Cartoon Network, which is part of Warner Bros. Must of been part of a working agreement with them and Lucas' crew.

~~/o/

Fox had so less trust in the success of Star Wars, that they gave Lucas both, the movie & merchandising rights, so they have not to pay his salary. Thus he had basically full control over the movies, I think only the live action movie distribution rights stayed with Fox. So Clone Wars could be basically distributed by anyone, Lucas made early on a deal with Warner for supporting the show.