That has always been part of the contract you have with them, and was widely criticized when the Amazon Android Store first came out.

It is my understanding that they guarantee a minimum amount of money to you, even if they give it away for free. (Which means they pay you when they give it away.)

They do compensate you for the special promotions. I'm not sure of the percentage but being the free app of the day and getting paid doesn't sound like a bad deal. Unless you are really intent on selling your "I'm Rich!" app to the 10 people you want to buy it.