I think you're misunderstanding where many of these ads _are_. They're on Amazon itself. Search for anything on Amazon, and you're going to see sponsored listings in the search results. Look at a product page, and you'll see competing offerings.
I've got some novels up on Amazon and it is totally accepted within the indie author community at this point that Amazon ads can absolutely be worth it.
I've got some novels up on Amazon and it is totally accepted within the indie author community at this point that Amazon ads can absolutely be worth it.