hosting & review are just the beginning. They also maintain substantial support around the App Store. So their costs will scale with number of downloads, usage, purchases, etc, which is why their fee scales with it too.
The paid apps also subsidize the support and distribution of free apps. This makes the App Store and platform more attractive and helps developers in the end OTOH, it is pretty wild that Facebook has never paid a dime to Apple for software distribution, aside from $99 a year.
