Fundamentally I believe this should be an anti-trust violation. But then I think the whole "only allow your own software to run on your patented hardware (which you therefore have a government granted monopoly on)" should be an anti-trust violation too.
Taking it as a given that Apple has the right to control what software people run on the IPhones that they own. And taking it as a given that we have apparently decided aggressively deplatforming these people is the right action (which I also disagree with in general [1] - but my objection doesn't actually apply in this case). I'm not sure that I see why Apple controlling it's platform is different than reddit/facebook/twitter/youtube controlling their platform, and if there is no relevant distinction then this seems like reasonable corporate speech.