My feeling is that the UFC went forward with it to keep Rampage out of Bellators hands. Bellator was gaining some steam signing these older names and putting them in fun fights. If they had announced Kimbo v Rampage it would pull a decent amount of eyeballs, regardless of the state of their respective careers. You can then use that pull to put eyeballs on your real product, if you load up the card with good upcoming fighters.
Once Rampage came to the UFC they saw an opportunity to a stop to, or at least put a hold on, that kind of scenario. Since during the ensuing legal battle Rampage wouldn't be able to fight for Bellator, putting him on the shelf for an extended time when the curtains are already coming down on his career. So even if they lose the case, it was a win for the UFC. The way it has worked out is probably the best case scenario for the UFC, since they got Rampage to fight for them, and will probably still lose the case and not be lumped with an expensive rampage for the rest of his career.