And this isn't just greed. If you look at the quality of Apps on the App Store it's very often quite obvious who is building for the platform and who doesn't understand the platform and is just trying to cash in on it.
Apple wants high quality apps, not apps from people who just want to cash in. (And the people who aren't willing to spend $600 for a mac mini because they're being cheap are not likely to invest the much more expensive development time to make a good app.)
