I just got the same letter. And you are the first to write about it. I guess you are right. I wasn't making any profit off of the Barnes&Noble affiliate program with Google Affiliate Network anyway. And right now I can't even login to the Google Affiliate Network. Just gives me blank screen.
Never having been with Linkshare I would probably have zapped the email as spam if I hadn't seen this thread first. I used B&N as an alternative to Amazon.co.uk for US visitors but never had a sale. It was only worth thinking about because possible commissions would be added to my Adsense income.