Affiliate links are OK according to google's TOS and FAQ, whether they are graphic or text based.
Maybe they are trying to limit bad feeling from their mates Amazon, and many other affiliate networks. Adsense may very well shake up the ad market if it gets critical mass and manages to keep its returns for publishers, and some noses are going to be out of joint as google, who before they loved, suddenly moves into their territory and threatens to affect their bottom line signficantly.
People ARE scared. Google is capable of changing the economics of on-line advertising, initially by offering a better deal to both publishers and advertisers from their reach and knowledge of sites and domains etc.
As evidenced by the description of one of the Adsense threads on the WebmasterWorld front page, many in the industry right now are spreading the message/belief/hope that Adsense wont last long or is at it's "pinnacle". Its about all they can do without looking long and hard at their commission structure and revenue models. It may happen, or it may not. Let's wait and see...