The issue is whether it is anticompetitive for a firm to block its customers from voluntarily purchasing products from its rivals without its consent, whcih is only given on terms that make it impossible for those firms to run profitable enterprises. That is unequivocally anticompetitive.
In this world, when someone acquires property (such as an Iphone), the manufacturer's preferences as to how they use that property are irrelevant. Modern doctrines ("first sale") and ancient ones (Blackwell On Property) make it clear that the owners of a product have no duty to consider the preferences of a firm's shareholders.
Apple knew this when it created the Iphone. No one forced Apple to sell people phones. The fact that some of those people want third party app stores (no one except the owner of an Iphone can use a third-party Iphone store) is unsurprising. Apple should not - and will no longer - be able to abuse copyright and cybersecurity laws to interfere with the property rights of its customers.
The people who bought Iphones may have been aware of Apple's preferences but they had no duty to respect those preferences - and Apple has no legitimate authority to force them to.
If I sell you a book and make you promise not to skip ahead to the end of the book and find out the ending, that's my problem, not yours. If I can't abide the idea that someone might use a product I sell in ways I dislike, my remedy is to go into another line of work - not find some way to reach past the sale of my product into the private affairs of my customers to exert control over them.
A phone manufacturer who only lets you run software of their choosing is no more just dishwasher manufacturer who only lets you use authorized dishes:
https://www.theguardian.com/technology/2015/feb/13/if-dishwashers-were-iphones
