In your example, it's like if you bought a TV from Walmart, took it home, and then any content you wanted to buy on that TV had to have a 30% fee paid to walmart since....that's where you bought it originally? It doesn't make sense there, and it doesn't make sense here. When epic sells you a fortnite skin within fortnite, apple provides literally no value to this transaction, other than being a gatekeeper between two legitimate sides wishing to engage in a business transaction - so that's why Epic wanted to make sure they can process their own payments within the app(and the court has agreed).