Publishing console games isn't as simple as hosting a file on a server. There is significant testing and certification cost, assignment of liability, and much more. Are those costs worth it to Microsoft? Will they have to advertise the game themselves? Who handles the support calls (yes, there are support calls for games)?

It just seems like Microsoft said "Our personal pipes are full, but maybe one of our licensed partners has some bandwidth for this game." Extrapolating on the headline, it sounds like Microsoft's pipes are full of Xbox exclusive titles. Preferring exclusives seems like a reasonable general guideline when there are simply too many games worth investing in. This is especially true for simultaneous development of cross-platform games. Microsoft certainly wouldn't want to spend time and money waiting on or helping a studio finish up the Playstation 3 port of their game (PS3 development always takes longer).