Spotify is just calling it unfair now because Apple has just now started to reject their updates on the basis of this rule, even though Spotify didn't change anything related to this in the update in question.
There is no technical reason to not allow an app developer to use an alternative like Stripe as their payment processor. Apple tries to sugarcoat it by using phrases like "seamless integration" for end users, never having to enter their credit card number. The reality is, it has nothing to do with user experience. It is 100% about getting that 30%. Nothing else.
I accept a certain level of a walled garden by buying into the Apple ecosystem. I absolutely do not condone charging developers 30% for payment processing. I buy apps from the store, and developers get shafted just for that. I have never, and will never, perform an in-app purchase without first checking whether I can pay the company directly. I don't care if it's $9.99 in-app, and $9.99 on an external web site. I will ensure that I pay outside the app whenever possible. The problem is that the majority of users do not do this. And they shouldn't have to.
And of course Apple has iBooks and doesn't have to worry about losing money on each sale because of a 30% transaction fee.
You are arguing that Walmart shouldn't sell their own brand of soda because it charges Pepsi some percentage to sell theirs.
Ridiculous.
I was just responding to the original comment that made an incredibly childish argument agains the owner of a store not being able to compete with products from other companies.