This is my theory on how this is running.:
(a) they can do it legally because it's done on a per-search, not per-click basis
(b) they make it vague enough so that people think you have to click to donate
(c) they reap big rewards without getting in legal trouble
It's a jerk move, if my theory is correct, and at best very poorly thought out if my theory isn't. It's clear that advertisers, who scrutinize the terms more than anyone, are confused -- how do you think regular users will react?
<addendum>Let me also take this opportunity to mention that, as a person who just used it for the first time in a couple years, AskJeeves really sucks as a search engine.</addendum>