Why does developers must pay a fee to link to an external payment service? Developers are not using anything from Apple. The app wasn't sold for a penny, it was free. They are not using the App Store for a global distribution of their apps. Not using Apple's SDK to develop the app. Not using any of Apple's security tools. Not using Apple's code validation and safety. Not using Apple's tech support for the OS and hardware. Not using Apple's update on the users' devices. Not using Apple's hardware to earn an income. Not using Apple's marketing of their own products and services to provide a solid user base for their apps. Not using anything of Apple's ecosystem of wearable, mobile, portable, vehicular, residential or computer based.
That is just Apple's immense greed. Apple should've been paying app developers, as they are the sole reason there are billions of Apple devices globally. Burn, Apple, burn.