Google AdSense Referrals

I'd recently decided to try using Google's AdSense Referral ads. These ads paid you for each action, rather than per click or per impression, so had the possibility of generating good returns. Then, I received an email from Google saying they were going to discontinue the referrals program during the last week in August. Even though I've read the AdSense blog, I'm still not really sure why they're doing this. Perhaps poor performance?