From a very interesting interview here from Peter Vesterbacka, Maker of Angry Birds:
“Apple will be the number one platform for a long time from a developer perspective, they have gotten so many things right. And they know what they are doing and they call the shots. Android is growing, but it’s also growing complexity at the same time. Device fragmentation not the issue, but rather the fragmentation of the ecosystem. So many different shops, so many different models. The carriers messing with the experience again. Open but not really open, a very Google centric ecosystem. And paid content just doesn’t work on Android.
So Angry Birds has to be ad supported on Android because you can’t charge with Android. Ah the great race to the bottom. The one caveat is that they estimate to make close to a million dollars a month on the ad supported model.