My guess would be that it was personally advocated for by someone who has enough influence within Apple to make it happen. Possibly someone on the hardware team, as I hear that the people developing the Apple Silicon processors run linux on them while they're in development.
This used to be one of the best things about Apple when Steve Jobs was still running the company: you'd get a bunch of features that a purely profit-focussed "rational business" would never approve, just because Steve wanted them. And I suspect Apple still has some of that culture.
On the internet it seems antitrust law can just be used to explain every. Antitrust actually has a pretty strict legal definition. And not a lot of thing fall into that. And if it Antitrust did apply, it would apply far more to the IPhone.
It would take an outright legal revolution in the definition of antitrust for this to be even a remote possibility, and frankly that is not happening.
