EDIT: I can't find the thread where somebody said Denny objected, so I have no idea how he feels about ZK and his music.
Considering that denny schneidemesser has said he doesn't like his music being used in this way, it'd be nice to see music in ZK from an author that enjoys its inclusion.
Denny released his music under CC-BY-NC-ND, the most restrictive CC license - I think it's fair to say his intent was that people copy his music around freely but primarily enjoy it on its own and not attached to a video game.
Particularly since ZK does offer products in return for money (even if it's minimal cosmetic items and barely covers server costs) it could be argued that this is breaking the spirit of NC.