I think one way to look at Apple's iOS App Store is like a mall. Storefront property in a mall must be leased from the mall, it's not available on the open market. Shop owners pay a lot for access to the large amount of foot traffic the mall provides.

Developers are free to take their business elsewhere, such as to Android or PCs, or even to some open source phones. Apple just happens to own the "mall" with the lion's share of the foot traffic and the wealthiest customers.