That section doesn’t mention monopolies, it’s mentioned earlier in section 2. Not section 45 (which sets up the FTC).
But fine, we’ll go with your way, throw whether or not it’s a monopoly out of the picture (BTW the link you share does not say Apple has over 50% market share, and if it did, still not a monopoly).
You’ll note if you actually read the section this part:
“The Commission shall have no authority under this section or
section 57a of this title to declare unlawful an act or practice on the grounds that such act or practice is unfair unless the act or practice causes or is likely to cause substantial injury to consumers which is not
reasonably avoidable by consumers themselves”
So something a consumer can reasonably avoid isn’t unlawful. Can a consumer reasonably avoid using the App Store? Yup. Buy an Android phone. Which are available cheaper than the iPhone, and in more options. The consumer has choice. Cheaper choice. There is and remains competition in the smartphone market place.
Don’t like what Apple offers? Buy a different phone. It’s simple. Just because you want something doesn’t mean you are entitled to it.