It's pretty obvious in the documentation and on every page except for the original product announcement that it's ad supported, but I don't expect Amazon to use a bunch of their 200-word product announcement to justify why there are ads.
The important point is that it's free, meaning I don't pay for it with dollars. When is the last time a free product shouted "BTW it's only free because of ads?!" I felt like it was implied, and I don't expect marketing to be like, "And here are the tradeoffs you have to make and that you might not like." When you download an app from the current app store (Apple or Google) it says "Free," even if the app includes ads.
I don't feel slighted one bit.