There is a batallion of snitches and "researchers" on the web! Frequently we receive inquiries for booking links at our sites ... "one of my clients is interested in advertising on your site ..."
Loads of researchers are smarter than this to provide Google and all others with any data they require for decision making.
Of course, they can not create such complex algos to cover that issue nor can they admit to have any such data.
But a nice Email accusing a site that Google does not like to see on top can be manualy banned or shifted downwards.
In case things go court they can produce some user complaints and Google would look again like very good boys protecting the web.
Reports from their own snitches would damage the image, better encourage user snitches, that's web 2.0 :-)
Those sites who buy links have MONEY. Kill all linkbrokers and those sites will have to shift their money into adwords.
And as long as Yahoo and MSN can not fully compete with their ad networks, most cash will go into Google ads.
Smart, isn't it.