The crucially important subtlety here is that Apple requiring developers to use the App Store doesn't leverage an existing monopoly (like what Microsoft had with Windows).
Compare the games console market. Nintendo is allowed to say you have to go through them to sell games for the Switch, ditto Microsoft with the Xbox. Sony doing the same thing with the Playstation is exactly equivalent, but they're approaching the sort of market dominance where it might soon be illegal for them (and them alone) to do that in some markets.
The crucially important subtlety here is that Apple requiring developers to use the App Store doesn't leverage an existing monopoly (like what Microsoft had with Windows).
Compare the games console market. Nintendo is allowed to say you have to go through them to sell games for the Switch, ditto Microsoft with the Xbox. Sony doing the same thing with the Playstation is exactly equivalent, but they're approaching the sort of market dominance where it might soon be illegal for them (and them alone) to do that in some markets.