I agree that it's inconvenient for users; it annoys me every time I have to bounce out to the Amazon website to buy Kindle books, for example.

But I don't see the alternative from Apple's perspective. If they allowed revenue side-channels, every app would be free, and useless if you didn't purchase an expansion pack from the developer.

Sure, Apple could run the app store at a loss. They could allow side-loading. They could do a lot of things. But I don't fault them for declining to.