The action that triggered these events is that Epic chose to sell some items at a higher price via the Apple store to cover the 30% "tax" and make it clearer to consumers that they are paying a premium to the middleman, or at a lower price via in-game currency (which could be purchased elsewhere) directly in the game.
Offering this consumer choice is what lead to Fortnite being banned.
Steam isn't manufacturing computers. This is about Apple the computer manufacturer. It's unfair for them to lock their computers/phones down so people can't install software without Apple's permission.
The App Store is more or less a red herring here (relevant only to the 30% fee being too much).
Is it though? There's no legal precedent that would allow for software vendors to force device manufacturers to allow their software to be run on the manufacturer's hardware. Otherwise we'd see Android on iPhones, legally-mandated Facebook integration on everything, custom car firmware etc.