I'm not sure how one concludes that getting $50k of free money is even a smart assumption to make. So they give away X amount of money per day to app developers because they are being nice?
When the FAQ appears to address something (selling at less than list price) and doesn't call out an exception for free, it's not an assumption. Companies do all sorts of things. I don't understand all the economics of amazon's business model, but I can read.
I would interpret the sale price of the app to be zero, so they get 20% of list.