One, I don't think they consider being given a fat wad of cash by Microsoft to develop extra content for the 360 version to be the same as being "Microsoft's pawn." (I'd imagine that's what happened.)
Two, no one really has any way of knowing what releasing extra content for the PS3 is like. I mean, almost everything that's been released on Playstation Network so far has Sony's mark all over it. Maybe the Xbox Live! method is just that much easier?
Third, how can you possibly call them a pawn of Microsoft? I mean, Rockstar released how many games with some level of exclusivity for the PS2 console? I can't even begin to imagine how much good GTA3/VC/SA/Bully did for Sony. Not to even mention LCS/VCS on the PSP.
Chill out a bit. If I were you I'd be more upset that developers are using Xbox Live! as an excuse to release unfinished games for $60 a piece, then charge a fat wad of cash for players to get the final pieces a few months after the release date.
Well, there's several possibilities you may not have considered.