The key point I presume is the definition of "market" here; and I'm not convinced the market "software that runs on iOS" is so large that monopwhatever can be declared there and used to justify price control. Though as a thought experiment Apple is like a wealthy landed gentleman who sets up a well maintained outdoor market for the tradesmen to bring goods and the lower classes to buy them. If Apple owns so much land…
Total iOS app store revenue is somewhere within the neighborhood of $50-100 billion per year. That puts it roughly on par with the US's cable television industry.
There are about 100 million people using iPhones in the US alone. That's about twice the number of cable TV households in the US.
So, it seems like the market for iOS apps is similar to the market for cable TV, and there's also less competition, as I may have other options for getting TV shows, but I don't have any other options for getting apps onto my iPhone.