"I don't believe it. Everyone would click on their own ads."
(Note similarity to the Starlite teacher's answer.)
So I wonder how many people, if they really stopped and thought about how online advertising works, in particular pay-per-some HTTP exchange, would identify the flaw? And of those people, how many would be concerned about potential loss of advertiser ROI?
As an aside, I find it interesting that my friend "got it" right away (after the payment model was explained to him). In the continued vein of my wanting to understand how people solve problems, I wonder what factors go into developing an aptitude for identifying security holes, backdoors, etc?