IMO this is an example of exclusive dealing arrangements and restricting free trade.
15 USC Code 1 Trusts..
Every contract, combination in the form of trust or otherwise, or conspiracy, in restraint of trade or commerce among the several States, or with foreign nations, is declared to be illegal. Every person who shall make any contract or engage in any combination or conspiracy hereby declared to be illegal shall be deemed guilty of a felony, and, on conviction thereof, shall be punished by fine not exceeding $100,000,000 if a corporation, or, if any other person, $1,000,000, or by imprisonment not exceeding 10 years, or by both said punishments, in the discretion of the court.
From that, $100 mil is a slap on the wrist, wait, a mosquito bite for Apple.
https://www.law.cornell.edu/we...
Exclusive dealing agreements require a retailer or distributor to purchase exclusively from the manufacturer. These arrangements make it difficult for new sellers to enter the market and find prospective buyers, thus depressing competition. However, because companies widely-use requirements contracts, which essentially are exclusive dealing agreements, for purposes that promote competition, exclusive dealing arrangements only face rule of reason scrutiny..
Section 2 makes illegal a firm's refusal to deal with another firm if the refusing firm refuses for the purpose of trying to monopolize the market. Meanwhile, section 1 prohibits a group from refusing to deal with a particular firm. A group refusal to deal is known as a group boycott. Because of seemingly contradictory Supreme Court decisions over the years, the question of whether group boycotts are subject to the rule of reason or a per se rule has been left murky.
Apple with it's walled garden can certainly dictate who's allowed in but I think there could be legal grounds for challenging that in court. Sure Apple can say "we're protecting our customers" but at the same time they're restricting competition and free access to markets, namely the app store.