Apple's letter to developers
The App Store is not simply a marketplace – it is part of a larger bundle of tools, technologies and services that Apple makes available to developers to develop and create great applications for iPhone, iPad and other Apple products. We know you knows this. You have been a major beneficiary of this investment and support. You have made great use of Apple-provided tools, such as TestFlight, VOIP, Stickers, iCloud document storage, ARKit, Messages Extension, ReplayKit, and Push Notifications. To highlight one example, for years now, you have used Apple’s groundbreaking programming language, Swift. […] Apple doesn’t charge separately for the use of Swift or any of the other tools that you have used to develop great apps on iOS.
Not only has Apple supplied tools and technologies for you to build your apps, but it also provided a marketplace—the App Store—to help make them a success. Because of the App Store, you have been able to get this app and other apps into the hands of millions instantly and at no cost, as Apple charges nothing upfront to distribute apps that are free to download. […] Of course, you could not have achieved this success without great apps, but it nonetheless underscores the value Apple brings to developers like you.
Still, you have many ways to reach consumers, including through Android stores, PC-based platforms, consoles (Xbox, Nintendo, Play Station) and your very own app marketplace.
Apple has hundreds of thousands of developers distributing apps on the App Store, and Apple is proud that it offers them all, from the student in her living room to some of the largest companies in the world, the same terms and opportunities.
The App Store is not a public utility.
Exhibit E, Email exhibits from Epic v. Apple.