They can likely write whatever they want but (at least in the EU) by law you cannot sign away your rights, so even agreeing to whatever someone write in TOS or EULA doesn't mean it will hold up in court.
So, 30% or 99%. Whatever it is. Apple put it there. It is legal. If you don’t like just don’t accept. Go distribute your app somewhere else (also, App Store does not qualify for monopoly)