Apple just kicked Fortnite off the App Store

I don't get the problem.

The App Store (Apple or Google) is a software distribution platform run by the owner of the platform. The platform has rules to cover costs and generate profit. Participation is entirely voluntary.

Epic Games has violated the TOS of the platform, and for this both Google and Apple has removed their app(s) from their platforms. Again, participation is entirely voluntary, but if you participate you're expected to follow the rules of the platform.

It's really not down to Apple keeping a walled garden, or Apple vs. Users, or Google spying on you.. again, Participation is entirely voluntary, that also goes for users.

Just because company creates and operates a market does not mean that they can act without restrictions or disregard competition law once the market becomes economically important.

I'm not very familiar with the US wording in the US law but EC defines relevant market as: "A relevant product market comprises all those products and/or services which are regarded as interchangeable or substitutable by the consumer by reason of the products' characteristics, their prices and their intended use"

A relevant market in this case is iOS App store and the payment system. Not smartphone market. Epic is not selling phones.

(I don't know where you get the voluntary participation argument, maybe that's some idealistic political viewpoint, but it's not the only thing that matters in competition law and antitrust)