https://developer.amazon.com/public/solutions/underground/do...
"Interstitial Advertisements
When a user launches an Amazon Underground app for the first time, a welcome message in the form of an interstitial ad plays. In subsequent times that a user either launches or resumes the app, paid interstitial ads will also sometimes play."
Interesting. So amazon deals with putting in the ads and the app dev gets paid for every minute the app is used. Seems like an interesting idea, probably useful for games that are very engaging (read: time drain)