This is a very good thought I hadn't considered. Thinking about it, on the one hand, I can imagine it easy to circumvent by switching ad providers. On the other hand this would drive many spammers to using alternative ad providers, which would degrade those services so it may be strategically good for Google. Or perhaps by driving spammers and affiliate marketers on to a competitor, it will help them acheive critical mass, something google would like to avoid. Also, using some kind of 'ad bounce through' ratio may have unacceptably high false positive ratios, again a bad outcome.

I hope this was not too much rambling, thanks for the interesting perspective.