It's mostly due to incentives. The most effective way to go after search engine spam is to aim for what the search engine spammers are after, which is mostly a

It's mostly due to incentives. The most effective way to go after search engine spam is to aim for what the search engine spammers are after, which is mostly ad impressions. If you get rid of the ads, you get rid of the search engine spam. Of course, that would be directly hitting Google's bottom line, and is never going to happen[1] The other factor is of course that they are very big, and get directly targeted in a way I'm not. [1] http://infolab.stanford.edu/~backrub/google.html#a http://infolab.stanford.edu/~backrub/google.html#a