Several AdSense publishers have received notice that their sites, which are mainly of the AdSense arbitrage/made for AdSense (MFA) variety, are being kicked out of the AdSense program as of June 1. According to Jennifer Slegg, the letter explains that the site involved has an "unsuitable business model," and most of the targeted publishers are earning significant amounts of money from AdSense. (Though how much of that money is being pumped back into AdWords to drive traffic to the site is not known, of course). Google will pay the publishers through the end of May.
Slegg notes that this could be good news for advertisers, since it should help clean up the content network:
From a business perspective, it does make perfect sense for Google to make this move, since so many Google AdWords advertisers refuse to advertise on the content network because there are so many "Made for AdSense" style sites as well as those doing arbitrage. So in the long run, it could mean more money for publishers if/as advertisers return to the content network.
In the short-term, this could affect publishers not engaging in MFA sites, since these large-scale arbitrageurs will take their ads out of the program. These publishers could see at least a temporary drop in earnings, but a better-quality content network could bring in more advertisers in the long run, Slegg said.
UPDATE: According to a Google spokesperson, this is part of an ongoing quality initiative on its content network: "At Google, we are always focused on how we can make the user experience as positive as possible while still providing value to our publishers and advertisers. As part of this effort, we continually conduct automated and manual reviews of publishers and sites that violate our policies. In some cases, violations of our program policies will result in termination from the AdSense program."
It's not clear how many sites were affected in this batch of reviews.