From the ease of which G was able to take down BMR and other blog networks, one has to think why G didn't do this a long time ago. Was it only because seomoz was publically outing networks that they felt forced to act? The timing could suggest such. There's the theory that G turns somewhat of a blind eye to BH, because it forces established companies, companies that don't want to use BH because they want their brand protected or don't know about BH, to pay for adwords. G knew about PBNs for a long time, and had the capability to take them down for a long time. They provided more link juice than almost any other method. One has to think why they waited. And for the people who think G is scared of negative SEO... G simply protects established sites from this. It's only established high ranking sites people would neg seo anyways.