> An example: if you do a Google search for "Amazon <some book>" then you'll almost always get an ad from Amazon. So Amazon pays for a click that would have happened anyway
This is absolutely understood, and in fact deliberate. Amazon does not want another advertiser sitting above their top position in the organics. It weakens mind share, and risks losing a click.
Ok, admittedly my example was bad. However, the replies are all basically assurances of the form: "Don't worry, a human at Amazon made that decision, so it's ok."
I'll repeat that ML is useful, and I'll add that it's useful in human systems, but its utility is constrained because for the really important stuff you eventually have to have a person check its work.
This is absolutely understood, and in fact deliberate. Amazon does not want another advertiser sitting above their top position in the organics. It weakens mind share, and risks losing a click.