> That’s like complaining that IKEA has a monopoly within IKEA stores.
I feel like the fact that only Apple makes iOS devices is what's confusing people.
Suppose that Tesla is the only company that makes electric cars and also the only company that has any electric car charging ports. (This is a hypothetical; in practice other companies offer both electric cars and charging ports.)
They don't have a monopoly on cars -- you can go buy a Volkswagen diesel if you like, and "electric cars" isn't necessarily a separate market than "cars" at the time of purchase. But once you have an electric car, they do have a monopoly on charging ports, which is a separate market because you can't run your electric car on petroleum.
This is easier to see if you add another electric car company but not another electric car charging company. So you go buy your Nissan Leaf but you still have to use a Tesla charging port with it. Clearly a monopoly on charging ports. But still the exact same charging port monopoly when they're the only company making electric cars.
You can have a monopoly on iOS app stores even if you're the only company that makes iOS devices. Having fewer competitors in the other market doesn't make you less of a monopoly. Even the fact that they compete there at all -- if Amazon was the only company with an iOS app store, they would have a monopoly on iOS app distribution even if they don't even make iOS devices.