But does "belong" matter when it comes to distribution? If Dovetail or Night Dive or whoever has true distribution rights, then they would be the ones to decide what platforms a game comes to in most situations...
Maybe it does, maybe it doesn't. We don't know the fine details of this specific licensing deal. Hell, we don't even know if the crapton of DLCs was Microsoft's or Dovetail's idea (I wouldn't be surprised if it was Dovetail's idea, as they also offered a crapton of DLCs for their Trainz games). We also don't know if Dovetail has the full publishing rights for everything called Microsoft Flight Simulator X or just for this specific version.
Well, if you want to get down to the nitty gritty, we don't know much of anything about anything. We're all born speculators. I speculate that since Dovetail has distribution rights to MS FS X that they're probably the ones to talk to to get it here. I also speculate that they are the ones responsible for the crapton (great word) of DLC (because of their Trainz series, as you noted). And further speculation leads me to state that if a game with $520 of DLC came to GOG that there would be a complete riot by the user populace. :)