Others have replied, but I will also reply. Epic, years ago, clearly broke the rules of the App Store. Their account was banned years ago because of that. It's not a ban for "no reason".Apple not allowing Epic onto the US storefront is basically telling developers that Apple can ban apps for no reason in the US. Yikes. Very excited for Epic’s ads to not buy an iPhone if the millions of Fortnite players want to play on the go. Bad business decision Apple when it is another reason to buy an android over iPhone for most gamers.
Also, Apple can and always has been able to ban apps for whatever reason it wants to. Apple created and runs the App Store and has the final say in what apps can be in there.
If you ran a retail store, would you want to allow anyone to put whatever they wanted into the store? Would you want a government to force you to sell a particular model of television from a brand that was actively suing you over different but related matters? To make the analogy even more accurate, selling that television would likely lose you money because that other company is trying to encourage governments to force you to accept products to sell and not make money from them.
Don't think Epic is some innocent company here. It flagrantly broke Apple's App Store rules and then sued over it. It lost that suit (it's won part of a related one) with judges basically telling Epic to that it has no legal grounds for what it's doing.
I've said in other comments that I think Apple should allow 'sideloading' and other app stores. I don't agree with some of Apple's other actions. I do, however, strongly side with Apple on this particular issue with not allowing Fortnite. I used to be more sympathetic towards Epic, but over time I'm less and less sympathetic because of stunts like this.