That's what happens when Microsoft offer you a big fat cheque in exchange for a period of exclusivity on their console.
I was under the impression that Microsoft will say to developers, we'll give you X amount of money for a 30-day DLC exclusive and if you say no, we won't sell your DLC on our service.
Although, this information was gleaned from the internet, and as such, is most likely false. The trouble is that it's something I can see Microsoft doing.