No, but the apps are. Spotify comes in at 98MB, and Kindle at 113MB. That is a lot of bandwidth. Since Amazon don't offer IAP, they get that for free, Spotify do offer IAP, so contribute that way.
And no, Amazon doesn't get it for free, they pay the developer fees like ALL the other developers
They do; it's part of the 30% margin for the price of an App/IAP.