I'm no legal strategist, but I would take the position that even if Apple's store charged double or triple what others do, that is not the standard by which what is legal should be measured.
Apple (or any platform) provides a service for people / companies to voluntarily develop software to offer to the users of that platform. What cut they take is an agreement entered into based on people's assessment of the costs and benefits provided, and people are free to decline it and take their efforts elsewhere. Their terms for their platform are not hindering people from competing and offering services on another platform. Each developer is free to make his/her own judgement about whether that commission is tolerable and desired, for the value created.
If the agreement if free and voluntary, Apple could set any price it desires. You, in your own business are free to (and probably generally also try to) set the maximum price you desire. By some similar logic, all apps should charge the same price to be seen as fair. What principle says that one platform's price has to be in line with others? It's what your offering can justify, and what people are willing to pay, with free choice.
I wasn't making an argument that Apple isn't doing anything illegal, but I think it's not credible to argue as @herf did that "there has been no market discovery of prices due to exclusivity."
The market for Android and Windows app stores has settled on exactly the same rates as iOS apps, which is exactly same rate as game-console platforms that have only one exclusive store. 30%, across the board.
Android alone is sufficient evidence that sideloading doesn't force the dominant app store (Google) to lower its rates to compete.
Apple (or any platform) provides a service for people / companies to voluntarily develop software to offer to the users of that platform. What cut they take is an agreement entered into based on people's assessment of the costs and benefits provided, and people are free to decline it and take their efforts elsewhere. Their terms for their platform are not hindering people from competing and offering services on another platform. Each developer is free to make his/her own judgement about whether that commission is tolerable and desired, for the value created.
If the agreement if free and voluntary, Apple could set any price it desires. You, in your own business are free to (and probably generally also try to) set the maximum price you desire. By some similar logic, all apps should charge the same price to be seen as fair. What principle says that one platform's price has to be in line with others? It's what your offering can justify, and what people are willing to pay, with free choice.