It was discussed a couple of months ago in this thread:
I have a keyword phrase that's lucky enough to be featured every now and again. Yesterday I was stung for another 129 clicks (against usual low single figure clicks).
Clicks on a phrase link in a box labelled 'Web Offers' are not search quality traffic. The keyword phrase links sit on unrelated pages, so they're unlikely to be as good even as Adsense traffic.
Isn't it about time Google put a stop to this?