Patreon doesn't have a choice. Apple is too large to give up having an iOS app - too many customers will just walk away instead of using the alternative (or so they think - but I don't blame them for not being willing to experiment - if they are right it will cost a lot of customers)
Well there is a choice, but it is questionable if suing will or will not result in any change. Even if they win in court it will be several years and millions of dollars in legal fees, and it isn't clear they will win.
They can _not_ charge through iOS though. It’s a very small matter for me to navigate to the Amazon website to buy a new book, then load it up in the Kindle app.
I imagine they already crunched the numbers of doing the Spotify approach vs. kowtowing to Apple. Unlike Spotify and Amazon, Patreon does have non-negligible competition to consider who will take advantage of the more convenient payment.
