The difference is that certain consumers are limited to only buying products from one supplier, and that limitation is a wholly artificial one introduced by said supplier. The difference is a pretty gaping one. The only way it would even be remotely comparable would be if you put it in the context of a completely ridiculous of a made up situation where consumers have no choice but to buy products from McDonald's or face some other inconvenience compromise or have to make some other, longer-term buying decision.
Like, maybe if McDonald's made a deal with the city of Chicago such that they were the only restaurant allowed to have Drive-Thru windows. Then people who live in Chicago would have to either buy food from McDonald's if they want the convenience of a Drive-Thru, or they would have to go into a restaurant, or they would have to go somewhere outside of Chicago. Or they could just move out of Chicago and somewhere else.
Either way, such a scenario would absolutely provide a legal justification for McDonald's to have to sell food from competitors.
Uh, there's other app stores. Just like there's other restaurants. If Apple was the sole provider of cell phones, or the sole marketplace for apps, or McDonald's was the only restaurant, your point would make more sense. Just because Apple is the most successful doesn't automatically make it a public utility or that they're so large other platforms automatically can't exist. Indeed, Android exists. Now, if Apple was making closed-room deals with Google execs to keep prices at a certain point, that would be anti-competitive behavior. But simply creating a successful device and opening up an app store where people are free to sell or not sell their apps - I don't get the need for strong-arming them.
I also 100% agree with Apple that forcing them to lower its commission, allowing sideloading, etc. takes away from the device and software it has created.