> The government can not compel two companies to engage in a contract.

It seems like there ought to be exceptions to prevent certain gatekeeping behavior. As a very extreme example, suppose a company bought up a set of private roads in such a way that without their roads, people in a certain area could not access the rest of the country. One would hope that the government would compel the gatekeeper to do business with everyone! (Perhaps with some reasonable fees, weight limits, etc.)

Apple decided to insert themselves as a gatekeeper between iOS users and developers, so it shouldn't come as a surprise to them that deciding to "not engage" with some parties raises antitrust concerns, since it means blocking users and developers from engaging in mutually beneficial relationships.