There was probably a demand for it so they fulfilled the request. Why would it anger you that its for PS3? I'm glad these types of things happen. Final Fantasy XIII will be on Xbox 360. It can't only work one way.
If it makes you feel any better it was released on the PS3 over a year later I believe, if there’s demand it makes sense, if I was creating a game I would want as many people as possible to have the chance of playing it.
I'm a big 360 fan, mostly becuase it is the best online experience, but why wouldn't they take a visually stunning game and put it on the system with the best graphics on the market? It makes sense to me that its on PS3.