The real line is that Apple claims what they can. If Apple tried to claim 30% off furniture no one would shop for furniture on the phone. If they tried to claim 30% from ride sharing, people would get pissed off enough to switch to Android.

So why not ads then? Well Apple doesn't have insight into the ad spending, and they are not a middle man to the cash transfer. They could try to force themselves into that position, say "if you want to have ads in your app, you have to use our SDK for that". But if they tried such a stunt, then Google and Facebook, would become very powerful very motivated enemies.