Sparrow tried to position it like that, but that was not the real story: it was not Apple "blocking" them, it was that they did not want to put in the effort to setup a real push service, and instead wanted to run all the time on the device (using battery) to maintain an open connection. Which, of course, not having every app doing that is exactly the problem that push notifications, as implemented by Apple, were designed to solve.
Actually they don't want to access your email, so they can't run a push notification service. And Apple Mail.app does the pulling thing, so allowing to change default Internet app or Mail app would not cause a problem : another app pulls, that's it
