Apple is the guilty party, mostly for the 30% fee, if it was a smaller amount ...
It's not really about the 30%, though that does feature heavily. What it's really about is that if you buy an iDevice, then you can only install apps from Apple's store - there's no other route without jailbreaking the device which for the vast majority of users is not on the table. Thus Apple can act as a gatekeeper and effectively control what you do with the device you bought and paid for.
Secondary to that is the issue of parting providers from the users - the "no outside payments" bit. It's all designed to keep providers of services (for example newspapers) from being able to a) offer their own deals to users, and b) know who the users are.
And of course, turning this walled garden into a cash cow by taking a significant slice of everything.
There is a valid argument that 30% is a reasonable fee for some of the services provided. For a small developer, not having to worry about distribution, payments, piracy, etc, etc is probably worth that 30%. But the issue is that there is no choice - some businesses (such as Epic) might be happy doing it all themselves, but Apple won't let them.