The fee could be lower. They (the App Store owner) only needs to sign a blob. They (Google/Apple/Amazon) choose to host the blobs, and then charge the developer for it. They claim “marketing” too, but as far as I can tell, that usually involves a sale and the app owner has no choice but to sell their app for less without their consent. I think Apple sometimes has “curated” lists, but it’s questionable how “effective” that is since they don’t release much numbers. What is the ROI of getting randomly chosen to be on that list? Is it worth 30%? We will never know.