That logic is incorrect... because it is not black and white as you put it nor Apple is really re-selling anything.
Apple charges every developer for having their apps in the App Store, and such charge lasts for a year. That is the 'fee' to 'sell' apps with Apple.
Then comes the subscription model for which Apple charges that 30% (15%) percentage. Such fee comes after using Apple's services for the subscription model, but developers are free, and should otherwise Apple would be likely charge for becoming a monopoly, to redirect to their webpages and such.
Spotify is not stupid and does the clever thing: pay Apple for having apps, and try to avoid any subscription thru the App Store.
Obviously, if Spotify sells the subscription directly, they answer for it, not Apple. Apple can only answer for the download of the app.
Apple just wants to guarantee some money, which is fair for me as an investor of Apple, but it is not fair to force developers to give Apple always part of their profit if they can find a solution otherwise as I said, Apple becomes a monopoly.
If Apple wants more cash... simple, increase the fee per year, or price according to number of downloads... I'm sure the number of useless apps in App Store would go down and sadly, Apple would not be able to brag about numbers ... oh ****!