How do you explain that Apple gets 100% of its margins on selling books, movies, music and any digital goods but anyone else must sacrifice 30% of those margins

How do you explain that Apple gets 100% of its margins on selling books, movies, music and any digital goods but anyone else must sacrifice 30% of those margins. For a subscription based offering, taking 30% away is surely going to kill any room for a healthy positive margin for most companies, rendering the business useless on the app store. I see this behavior as fundamentally anti competitive but don't see this discussed in the trial too much. This should be illegal.