Earlier quoted context omitted.
Yes. Apple's rules oblige app vendors to not advertise or link to any external method of signing up for a service. So, not only can you can only use Apple's payment methods, but you have to lie (by omission) to consumers about their options. This is the market distortion of the App Store.
> Apple's rules oblige app vendors The rules only oblige app vendors if they agree with that contract. Users also only engage in doing business with Apple on a voluntary basis. That's not extortion, by definition.
What is unlawful, at least in my enlightened jurisdiction, is abusing market power to mislead the consumer, and even more so obliging other parties to do so.