It is a monopoly because they sold me a device, then tell me what Apps I can put on it. That does not work with any other device you buy. If they leased the device that would be different. But they sold it to me and therefor they are being monopolistic unless they do one of two things; 1) basically approve any app that does not hurt the device's security, or 2) allow side loading of apps outside of the Apple store.
Would you accept a TV that only plays certain TV programs? No. How about a guitar that would only play classical music? No. How about a car that only goes to approved locations, for example, that do not serve alcohol. No.
Now you might say that I bought the device knowing this so it's my own fault. But the Apple App Store rules are capricious, arrogant, and stifling. I did not know that when I entered the Apple ecosystem and the rules have gotten much worse with time.
With computer chips being put in everything now, we have to stifle this nannyish behavior before it pervades every device. Think about a nanny Tesla, or a nanny refrigerator, or a nanny toaster that only toasts full grain bread.