I used to work on a high profile old media site, and based on my experience there, I believe that the motivation is that it's easier to game ad metrics with stupid gimmicks this way. Approximately at the time I had lost hope and was looking for an exit, they were working on a new photo gallery that would count each "next" click as a new page impression. In short, it allows you to muddy the water around what exactly constitutes an "impression."