I tend to think that funding the game from the beginning of the project allows the developers to not have to account for multiplatform limitations, be more focused on pushing that hardware, and in turn make a better game. Paying for a game after it's been partially developed multiplatform from the get go (or from a game on a multiplatform engine) seems like it doesn't really benefit by pushing the hardware. When exclusivity is bought later on it seems like more of a 'steal,' and is taking away from a game that may have well ended up on the other platform.
Sony does pay for it too, I remember UT3 (though to be fair this was a better deal to us because they actually told us the 360 one was coming later) haze(who knows if they paid, it may have been done to sell off of exclusivity - which it did), and probably to some extent MGS4.
