Would you be ok with if someone is taking 30% + Taxes of what your earning?
If I use the infrastructure they built that enables me to reach those customers, why shouldn't I expect to pay a commission to Apple? Do you think Apple doesn't pay Best Buy a cut of the sales for every Apple product sold at Best Buy? Or you think Best Buy should provide the retail space for free?
If you want to argue 30% is too high, that is another thing. But situation here is Epic not wanting to pay Apple a commission in using the App Store.