Turning sustained DDoS attacks into revenue sounds like an intriguing business schemes.
My general point is: AWS is a business, and it operates as one. There are no hollywood style bad guys sitting there in cubicle dungeons on chests filled with gold thinking how to extract money, quite the contrary. It is understandable that customer cannot pay unlimited (from customers perspective) charges, but AWS pretty much incurs them, as customer being ddosed is consuming resources that would be otherwise be sold to others, or engineer time that would be put into developing new features and attracting new customers.