Apple's job isn't to help build the next Facebook app. It is to build the best ecosystem they can. And that doesn't come from (a) having less exclusives or (b) having apps that don't take advantage of the unique features of the platform.
It's similar to Nintendo. They don't really benefit from having the next generic COD game on their platform. They benefit from having a game that makes the most innovative use of the touch screen control.
But why would you be hostile to someone just because they also built their app for Android? Successful people generally grow to encompass as many income streams as they can, and you're losing the long game by not reaching out to the next Facebook. Do you really NOT want that app on your store any more just because it's on the Play store as well?
It's similar to Nintendo. They don't really benefit from having the next generic COD game on their platform. They benefit from having a game that makes the most innovative use of the touch screen control.