Being utterly cynical, this is what I think is going on:
Google's search index quality has been negatively affected by Panda.
The Panda updates have damaged good sites and now PPC parked pages are creeping up in the SERPs.
Because Google's blind crawling is inelegant (crawl everything and then figure out what's good), tens of millions of PPC parked domains have been included in index.
The list of AfD domains can now be used as an additional filter for the SERPs. (Google announced before Christmas that it would be deemphasising PPC parked domains in the SERPs).