They're doing about as much to facilitate my use of Patreon as Apple is.
This isn't like a mall at all. This is like a web browser, where apps are webpages, and Apple is insisting that the contents of that webpage are something they can dictate all payment terms on.
For the airport analogy to work, it would have to be that you go to the Airport, go into the electronics store, buy a Kindle, and then the Airport insists it can take 30% not just on the purchase of the kindle, but 30% on every single book you buy on the kindle forever.
Apple taking a cut on the purchase price of an app that a user found via the app store does make some sense. Apple taking a cut of an in-app interaction with a creator that the user almost certainly found elsewhere is nonsense.
What next, should apple take a 30% cut of my rent because I found my apartment on the Craigslist app? Should they take a 30% of my train ticket that I purchased using the Safari app? Why does Patreon have to add a 30% cut on in-app content, when Safari lets me pay for in-app content with my credit card without taking any cut?