They are not a "private company" in that sense. With their user segment, they form a market and the app store establishes a "legal" system. Basically their user

They are not a "private company" in that sense. With their user segment, they form a market and the app store establishes a "legal" system. Basically their users are citizen of a Apple nation and whoever wants to make business with them, has to follow the "legal system". Consider the free market system (as the economic system of the western world), where you have choice between offerings, then the Apple ecosystem is the exact opposite.