Yet Apple don't charge a distribution channel cost if your app is free-at-download. How does that make sense? The backend costs to them are the same. In fact they're likely to be higher for free apps due to the many magnitudes more downloads versus subscription apps.

Apple want to have it both ways: the thriving ecosystem of free apps that promotes the platform AND their cut of subscription apps. Legislation that said that all app publishers should pay a flat fee per download would seem the fairest way forward.