For the same reason antitrust laws exist. Monopolies need to be prevented to maintain a healthy economy.

How is Apple a monopoly? They own a system and manage the rules around it, that is not a monopoly. Will you force car manufacturers to let you install any system in the car? Why can't I install any app on my car?

The same way Hollywood studios that controlled the movie theaters. Studio monopolies were teared down and Apple's monopoly should be teared down too.

https://en.m.wikipedia.org/wiki/United_States_v._Paramount_P....

Because it is acting against the interests of the users.

Would you still support Apple's position if, say, Apple would roll out update to iOS with new EULA, which would require you to give up your firstborn son for a Satan sacrifice?

Of course, you can refuse, but the device that you supposedly "own" would effectively stop functioning (obsoleted iOS devices lose their usefulness very fast: try doing anything with iPhone 5).

I would sell it and buy a new phone. Do you think Apple competitors would not pay you or accept the phone as part of payment if Apple did that? The problem is people think they are smarter than everybody and can tell you how to manage things. The market is smarter than anybody or any group of people. You cannot force a private company to cater to your beliefs of how a system should work. People buy iPhones knowing there is only one app store. And, as an iPhone user, is an advantage. I know that my grandparents and parents using an iPhone will not screw it up, or at least the possibility is smaller compared to the alternatives.