Sony does the same, it's just a different aproach. Like has been said here, Sony just prefers to fund in it's entirety projects like Heavy Rain or LBP, but you have to understand those companies could never afford to develop something like that. It's a risk, but it will still be exclusive even after years.
Microsoft, on the other hand, plays it safer by paying top developers who don't necessarily rely on that cash. The flipside is that since they don't exactly rely on that, most of those games will come out on PS3 anyway, just later, or much later.
In a way, you could say the Sony aproach is actually more limiting to everyone, since those titles will never, ever, release outside the PlayStation brand.
Either way, the thought that only MS pays for exclusivity is idiotic, Sony does the same, the difference is they publish most of it, while MS just secures X amount of time from whoever publishes it.