Okay, I'm now remembering those details, and reading the summary helps too. What I recall now is that there were plural issues, not just one. The hardware tying was only one thing. With Apple, if the only thing they're doing is limiting use of their software to "Apple-Approved" hardware, then I'm not seeing any anti-trust issue without some type of significant market share by Apple.
The thing to realize here is that M$ had problems because of their market share in combination with several "predatory" types of conduct.
Okay, here's a good way to look at this. An antitrust violation depends on two factors: one, market share; and two, how bad the conduct is. So, if one has a large amount of market share, then slightly bad conduct can be an antitrust violation. Medium market share would require conduct that is more egregious. And little market share would require really bad conduct.
Okay, this is only a practical way to look at this, not an exact rule of law.
When M$ was having trouble, their market share was pretty significant. Not monopoly power, but clearly the market share leader in an oligopolistic market. Coupling that with lots & lots of predatory conduct ("predation"), there was no doubt it would get popped.
Here, Apple's market share isn't too strong, is it? So, it's conduct would have to be VERY bad. I'm not so sure limiting their software license to hardware they "approve" would run afoul of antitrust laws.
Did this make sense?