Apple is not a monopolist. The judge in the Epic Games suit even said so[0]. It also isn't a public service. It's a private service operated by a private company where you have to pay to play. And part of paying means agreeing to certain terms and conditions before you're allowed access. Violating that agreement is grounds for termination of your access.
[0]https://www.npr.org/2021/09/10/1036043886/apple-fortnite-epi...
Apple is, in fact, a public company.
As a US-based company, there are many things that Apple is not able to do——for example, they cannot refuse to sell to Black people, despite violating Apple's freedom to "associate" with whoever they want, for whatever "private" reason they may have.
The US government can decide that Apple must provide equal access to all of its citizens, and in fact, the 14th Amendment and the various Civil Rights acts that it enables seems to require that every company that sells "to the public" treat members of the public equally.