How would Apple's policies be illegal? Epic entered a contractual agreement with Apple and then knowingly and intentionally broke it in order to benefit themselves.
I'm not a lawyer, but presumably because there's US antitrust law, under which the courts may find Apple's contractual impositions to be illegal. They may also find they are legal, but if the court has any doubt they should grant Epic's request to stop retaliation.
I doubt this is antitrust. There's a zillion other places to sell a game (as Epic has shown). If you want to claim the entire market in question to be Apple products, I doubt that will get very far. Everything is a monopoly when you make your market hyper specific.
This isn't some hyper specific market. Mobile games are a $76 billion industry (and rising) with the overwhelming majority of users only having access through either the Google play store or Apple store.
$76B is world market. US law doesn't apply to a significant part of that.
Neither Apple nor Google is a monopoly in mobile games. By revenue Apple has about 60%. Check [1], search threshold to see what cases often require.
Taken together they would be, but then you cannot prosecute Apple and win without showing collusion between them and prosecute them both for collusion.
Epic is specifically arguing that Apple's policies violate the Sherman Act, the California Cartwright Act, and the Unfair Competition Law of California. For more information, refer to the complaint: https://cdn2.unrealengine.com/apple-complaint-734589783.pdf
If we sign an agreement where in exchange for my services you have to give me your first daughter, you can break the contract and refuse to pay, even if you signed it, because it's illegal.
In legal terms, a contract is an enforceable agreement; by definition if it is illegal or void or unenforceable, it is not a contract. Breach is a thing you do to a contract. Therefore you can't breach an illegal contract - you never had a contract.