Can you provide an example?
I've just checked 3 different apps which offer premium features. All of them go straight to Apple Pay, without showing any other options.
EDIT: okay, I think get it now. In-app purchases != taking payments in general. E.g. I have different options for payments when using the Amazon app but only Apple Pay when buying a pro version of a metronome app. It's in-app purchases that force Apple Pay.