Although they state other reasons in their email I guess it's because they took over DoubleClick and they do not want to have two things that are doing basically the same.
your patience during this transition and here are some alternative
options to consider:
* Google Affiliate Network: As part of the integration of
DoubleClick, the DoubleClick Performics Affiliate Network will now
operate as the Google Affiliate Network for advertisers targeting
users located in the United States. Similar to the AdSense
Referrals program, the Google Affiliate Network enables publishers
to apply for advertiser programs and get paid based on
advertiser-defined actions instead of clicks or impressions. For
further details, please visit:
* AdSense for content ads: If you have less than three AdSense
for content ad units on a page, you may wish to replace the
referral ad units with standard AFC ad units.
It's a shame as the AdSense stuff is much setup better for international publishers as myself. E.g. it appear that DoubleClick only does bank transfers to US (business) banks.
Ah well, it was nice while it lasted. Guess I have to start working for a living now - or find some additional referrals.