The tone of Batman Returns lead to the studio taking the more light-hearted kid friendly approach to the two films that followed. Forever still held some darkness through-out and still felt in the same universe as the first two. The 4th film was just a cheese ball campfest.
I think the studio was willing to do anything to get Burton back since Batman did so well. They weren't prepared as to how far he would take it, and even for me, being loving the dark side, it was just to Burton for me. I actually prefer Batman Forever to BR. So I guess I should thank Burton in a way.
To be more exact, I don't think that Warner Bros. really thought much about the exact tone that Tim Burton was aiming for in Batman Returns, as long as he raked in the dough. When Returns made significantly less than the first film, Burton became the immediate fall guy for not making a more "mainstream" assable film. McDonald's was forced to pull a Happy Meal promotion when parental groups started to complain about the overly dark, violent and sexual content in Returns.