There is free to play shit on 360 and MS themselves have said they will embrace F2P openly (Google it; they've said as much during the release window of Happy Wars and other game companies like Epic have come out and said MS told them the same). If I had to guess, MS just doesn't want to allow new F2P games on 360 this close to the One launch. They maintain their own servers, unlike Sony the last couple generations, so it makes sense that they wouldn't want to take up any of their infrastructure with a platform they won't be aggressively pushing soon.
I'm as bummed out as any other Xbox fan that Core Fighters likely won't launch but maybe that stance could lead to a release on One. I want to play DoA on that excellent looking new controller.