Related to this, there is probably a psychological effect at play where people want to be consistent with what they have said in the past (see Cialdini and consistency theories), so just the act of asking the research participants to articulate their feelings about the person dug themselves into a hole. Talk less and you don't have to entrench.
True, the goal isn't to kick back money, but the goal is to refer people to Amazon that wouldn't have purchased from there otherwise. They have good fraud detection systems in place to make sure that people aren't just using the affiliate program to give Amazon discounts to themselves, friends, family.
Ultimately Amazon does what every company does in some form or another as it scales--cut incentives, trim margins, increase on-site advertising, etc. It's one long life cycle game that plays out again and again.
I've always assumed that app development was harder to monetize than typical paths to profit using standard web development + product/affiliate marketing.