Google is focused on weeding out spammers, scammers, manipulators, counterfeiters, cheaters and malware providers from it's network. Google has a dedicated anti-spam program for both it's organic search efforts and paid search programs. These bad advertisers also include shops trying to sell counterfeit products.
Last year, Google says it pulled more than 350 million bad ads from its network. This number represents a 59% increase from the 224 million bad ads the company removed in 2012. This efforts appears to be having a strong effect. The number of bad advertisers have been reduced from nearly 850,000 in 2012 to around than 270,000 in 2013. Google also claimed to have blocked 3 million bad ads from illicit pharmacies in 2012 and 2013.
Many of these advertisers have been thwarted by Google's safety screens and are searching for less secure targets and advertising networks.