Google Adsense has recently changed its adsense policy. The changes are mostly in ad placement section.
According to this new changes (actually backdated policy by 2 months to march 14th)-
A single referral button per product may be placed on a page up to a maximum of 4 buttons, in addition to the ad units, search boxes, and link units specified above. Referral buttons are considered to be ‘Google ads’ for purposes of these program policies.
Click here for rest of policy or here for rest of the story
Also checkout Google's Quality Guidelines