Actually, it's everybody's place to dictate them, through commercial negotiation. Antitrust generally applies where, as with the App Store, traditional market dynamics do not apply to constrain one participant's market actions.

The main complaint is over Apple's monopoly over the channel for getting apps onto your mobile device.

No, that's the foundation of the antitrust claims. The main complaints are that Apple is abusing its power over the app delivery channel to force other market participants to put up with a variety of things they would otherwise never agree to if market dynamics were functioning correctly, like giving up 30% of all their income even when it's unrelated to the app itself (e.g., ticket sales), or preventing competing apps (e.g., browsing engines, mail apps like Hey, etc.), banning third-party apps from using API features in the OS or from certain forms of outreach but letting first-party apps violate those restrictions (see, all Apple apps released in the past 3 years).