Amazon announced earlier this week that lockscreen ads won't be present on newly-purchased Prime Exclusive devices and an update is rolling out to the Amazon Offers app to disable them on existing handsets. If you don't want to wait, you can sideload that update right now.
When Amazon announced Underground, the remodeled Appstore that features the ability to play paid and freemium games (as well as other apps) for free, I immediately raised the question of how they planned to make money on this venture. Users obviously benefited by getting free stuff and certain developers would make more money since Amazon compensates them on a per-minutes-used basis. But where was the cash flow to Amazon?
A deeper dive into their developer documentation gives a clue. The following comes under a heading labeled "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.