As another commenter wrote, apple is providing a service beyond the typical transaction (like visa or mastercard) does. Similar to the gaming industry's 30% digital sales (playstation, steam, etc).

For small developers, like myself, this is great, because I don't have to put a large upfront capital to pay Apple to audit my app.

If 30% isn't the right number, what is the right number to pay for app store infra, payments, QA services, global tax audits and collection, global licensing limits, etc?