Weird. I always thought there was a shift in advertising from pageviews (3 years ago), towards Clicks per Thousends (CPM, now) towards Signups (Referrals, actually buying a product, 1 year from now) and Google and Amazon both had these kind of affiliate program's. Now Google is partly stopping with this ,a href="https://www.google.com/adsense/static/en_US/Referrals.html">referral

This is the case for referral units directing visitors to sign up for AdSense. After experimenting with this program over the past year, we've concluded that there are other products that are of higher value than this program to publishers in your region. As a result, referral units for the AdSense program will be retired in the coming weeks. Referrals to other products and services remain unaffected.

So they are only stopping the "adsense for adsense signup program" but still weird to see. The costs of running this service are next to nill (they already have accounting and billing in place) so why stop?