Seeing as he and Dicaprio got into a bidding war over this, I think Pitt had passion about the movie. This is just speculation on my part, but I think his wife's current health situation probably played a part in the studios getting greater influence over it. Who has time to fight the studio when you need to support your wife in such a massive decision (that has probably been in the works for quite a while)? If I were Pitt, I would have titled the movie something else and left WWZ in development a bit longer. I'm still going to see it though. It's probably going to be an awesome zombie flick even if it has little to do with the source material.
From what I read it didn't sound like he was fighting the studio over anything, it sounds like he saw the movie, and thought a lot of the political stuff wasn't working like they had hoped.