On practice, everybody was announcing at Google when it used the site's content to decide what to show, and kept announcing at them once they changed into targeting the user instead. The change went mostly unnoticed.

On the other hand, it can be that people detected the change on their results metrics, and decided to increase their spending because of the change. I really don't know how to differentiate this scenario from a normal increase on internet advertising that should naturally happen at the earlier days of a fast growing web. I don't think even Google (that has all the numbers) can tell them apart either.

This is a good point.

Also, this specific situation seems like a good candidate for regulation, which removes the need for businesses to be ethical of their own accord.