This is going to sound contrived, but they have a monopoly over the Apple App Store.
You might argue that this isn't the definition of a monopoly, but the DOJ gets to decide that, not you. And there is a case to be made that this is, in fact, anti-competitive.
Apple is forcing businesses that want to deploy paid software to pay their tax. You might say there are alternatives: Google.
Google does the same thing and should also be punished.
Before Apple started this extortion racket, there were ways to deploy paid software that could reach customers without having to go through a gatekeeper. Now there are very few ways to do so.
Both Apple and Google have undercut the web. You can't write streaming video software that isn't native. You can't have any sort of first class experience.
This is one of very few markets where there are no alternatives and the duopoly has actively done harm to the open alternative.
Is this Apple's responsibility or fault? Absolutely. And I remind my legislators of this.
If you're a developer that doesn't work for Apple or Google, you should feel a call to action to fight this asymmetric advantage they hold over us.