The problem with this analogy is that in many cases the "app" is only part of the service, or a way to access the service.

Lets say I sell a service where you can mine a bitcoin. It costs me $2000 in electricity for every bitcoin I mine. I sell the service for $2200. I offer an app, to access this service. Apple wants 30% of the whole amount, not 30% of the value-add that app brings.