As I note in my comment above, the App Store is not merely a neutral distribution conduit, it is also (by far) the primary discovery medium. The vast majority of app shoppers buy from the top lists. And the top lists are systematically structured to promote high volume, which causes devs to lower their price. That's why even the EAs of the world are calling on Apple to at least go by revenue rather than units.

The App Store is 'simply a delivery medium' only in the same sense that Fox News is 'fair and balanced'.