Fact is though it is easier safer and requires less details to be given to companies doing it through Apple.
I always like that argument... It always assumes everyone except Apple is a shady and scummy company that you cannot trust with your payment details.
When you pay by card (CC, Debit, whatever) in a local store, the payment processor they use also gets your payment data (obviously). If you don't trust a company with that, you shouldn't do business with them in any form. Even not through Apple.
That Apple takes a cut for payments through their AppStore is fine. But that they forbid the link to their own websites is kinda ********.
99% of customers are apparently not capable of finding the website. Most people don't even realize that it's the same. People really think they must pay through iOS to use the service on their iPhone and don't realize it's account based and works on all platforms.
If everyone knew that, there wouldn't be such a problem. Spotify could just add the 30% on top for the IAP to get the same output and the regular price on their website and people would find out. But people either don't or just don't care.
The other thing is, how Apple makes exemptions for certain companies and services (like Amazon video), but not for others.
IAPs also cannot give customers discounts... well.. they could. But it's weird to implement as you would need different SKUs for every discount level and each new IAP has to be "approved" by Apple.
For example: Student discount is not possible through IAP. You can only gather it through Spotify's website. And then, they're not even allowed to tell you in the App, because they're not allowed to link to their website for purchases.