take this instance, for example --
if the publisher simply created a page with no content except the keyword and the ad - and the user has no other choice to get out of the site except to click on the ad, how can you be sure that the advertiser is making money in the same way as the publisher? the user simply wants to get out of the crappy spammy page, with no interest on the advertisers product.
the advertiser does not want just a click - the advertiser wants a click on their ad only because the user is interested in whatever they are advertising. then, and ONLY then, will the advertiser make money.
the quality of the lead that the advertiser is getting is important to the success of adsense.