This isn't the first time there's been a public spat between Apple and a developer over in-app purchase prices. The fact that Epic is doing the thing where it's $9.99 in the game, and $7.99 on the web (or on PC, or whatever) is why Apple's mad. I don't think "Charge Apple users the same amount you charge everyone else" is an unreasonable stance.

You can debate the 30% thing all you want, of course, but the "charge all your customers the same" part is valid.