Spike was a wasting asset due to the weakness of the channel. They weren't growing forward there because pretty much anybody who came to Spike were already fans of the UFC. Moving to Fox gives them the chance to push the sport to a new, enlarged audience. Additionally they're getting almost three-times the amount they were receiving from Spike and the benefit of having the premier young males and sports broadcaster in the States as their media partner. There is also the strong potential for the deal to help the organization's global expansion through News Corp - they've already signed a deal with Fox Latin America. Its a risk undoubtedly but one well worth taking.
There are going to be prelims, some of which maybe aired if the main event ends early. The prelims will be live on Facebook as usual. I can't believe that anybody is actually complaining about this.