I understand your argument, but it considers that Apple is a philanthropic altruistic entity, that has only your best interest in mind when deciding for its "rules".
But, Apple (same as Google) are companies and not foundations. Their only "legal" purpose is to maximize the return on investment for their shareholders.
Take your example, and imagine that it is Facebook the gatekeeper of the unique store?
And if the App Store has so much "privacy" value, why do you think that people will accept to install another AppStore? So, there is no risk of the problem that you describe.
Still, if it is not the case, and people don't care, who is Apple to decide what is good and what is bad for people?
And you see positively their action mostly based on their marketing propaganda, ie that they protect and take care of you. But, for real, they will not hesitate to censor apps or features that are competitors to them, or that threaten their revenues.
And worse, so far they are all scared of possible antitrust issues, but otherwise, the day they will have all the market locked, they will racket everyone as much as possible.
Think about it, some people say that the current appstore "fees" are rights because they need to pay for the service and co. But now, look at Apple financials, their margins, and you can see that very few companies in the world have their level of wealth with hundreds of billions of cash pilling up!
Also, one day, the boss of the company can decide that anything he don't like (ex. Blue candies) will be banned. And the world would have to accept it unilaterally.
To conclude, I would like to paste this nice quote that I just saw on internet:
<< ‘The main effects of monopoly are to misallocate resources, to reduce aggregate welfare, and to redistribute income in favour of monopolists.’ (Harberger, 1954: 2) >>