A) MS has payed for content. we do not have the exact details on how much and exactly which games/dlc but it has happened. dismissing this fact is just stupid.
B) as much as I hate missing out on content, buying content is fully within MS's rights. We all know that Sony does it too (joker for batman: aa for instance).
C) as install bases increase for both platforms the cost of exclusive content probably also increases. i would assume that is why early on there was a lot of exclusive games and now there is a lot of exclusive dlc.
D) at least as far as MS is concerned, i would guess if anything they are buying up whatever resources they can to get people developing for natal. or should i say, if I were in charge of MS that's what I would be doing to help ensure natal sucess.
