The conspiracy theorist side of me thinks that Google doesn't want to get rid of all of the spam sites because they bring in revenue for Google through AdSense.

The conspiracy theorist side of me thinks that Google doesn't want to get rid of all of the spam sites because they bring in revenue for Google through AdSense. The practical side of me says that even 95% accuracy wouldn't be enough with 200 million websites out there. A 5% miss rate is 10 million angry webmasters potentially complaining to Google and requesting manual reviews.