If the average customer downloads 4 books per month through Kindle Unlimited, Amazon is essentially charging $2.50 per book. In this case, Amazon could afford to pay authors $2 per download and still make a profit of 50 cents per download. But if the average customer downloads 20 books per month, Amazon is effectively charging just 50 cents per book. Amazon isn't likely to pay authors $2 per download, while only receiving 50 cents per book; they would be losing money.