I guess I see both points of view. On one hand MS is trying to go small, use one carrier that is probably going to let them do as they want (regular updates) and see how that works. Try not to loose so much money catering to multiple carriers. On the other hand, the phones never really sold in any meaningful numbers so, again, why not downscale make the product available unlocked and sold by them. You control what happens to the product and you don't have to worry about carriers putting the device in the back of the store in the dark and discontinuing them after 3 months claiming they don't sell all the while giving incentives to the sales people for selling every other phone in the store.
If MS fails here they will know exactly why. This story sounds pretty familiar actually...remember when a certain computer company made an expensive phone and it was only available on one carrier...