I think GAN wants to transition all affiliate network merchants to Google Shopping exclusively. In my opinion, Google wants to be the only, affiliate in the Google search results. Keep in mind some Google merchants are paying commissions for Google Shopping sales, not PPC.
This is sortof the same logic applied when closing down iGoogle in favor of Google+. e.g. transitioning clients to the favored platform. I also think it is a flawed idea, in that now I'm on Netvibes, not Google+ and soon most merchants will have a Commission Junction, LinkShare, ShareASale, or Pepperjam account.