One of the illegal things under US monopoly law is tying--using your monopoly power in one market to enter another market by bundling the two products together and undercutting the competition.
Apple's push into payment processing here smacks very heavily of illegal tying to me, especially given how broadly Apple interprets competitor payment processing methods. In effect, the specific sin that got Fortnite kicked off was charging a lower price for using a company-specific gift card instead of using Apple's payment method. I don't see how that's not abusive tying.
Tying is actually not illegal and is common business practice if you don't have a monopoly. Apple started doing this all the way back in 2009, before they had a monopoly even from Epic's perspective. Therefore, the argument is that they aren't tying monopolistically because they were tying before they were even close to monopoly status. In which case, they haven't changed the rules, and also in which case the current antitrust law doesn't really address.
If you are following Apple vs Epic, even Epic admits this and says they can't pinpoint when Apple became a supposed monopoly, and argues that the tying is illegal because Apple is so large, not that it was illegal to begin with (because Epic admits that it would not have been illegal when Apple started). This is also why many legal experts say Epic has an uphill battle, because antitrust really only addresses abuses in monopoly power, not so much on what got you to that monopoly power.
Apple's push into payment processing here smacks very heavily of illegal tying to me, especially given how broadly Apple interprets competitor payment processing methods. In effect, the specific sin that got Fortnite kicked off was charging a lower price for using a company-specific gift card instead of using Apple's payment method. I don't see how that's not abusive tying.