Edit: I'm not saying I agree with the negativity. It's just an answer to the parent's question.
Leaving aside that I have to ask this question (WTF dude?) for a $12 book you get something like 60 cents, considering a 7% referral fee. And you can make less than that per the number of items sold, as lots of people end up buying something a lot less expensive, like an MP3 (since they buy from Amazon and are right there anyway), with the more expensive items helping to balance that. And I haven't sold any tractors.
So the total fee pales in comparison with the revenue Amazon generated from that article and if Amazon wouldn't do that, then the affiliate program would be basically a joke.
(edit: misspellings)
This is why online communications are though and no substitute for real world interactions - there's no body language to read between the lines and so you have to be pretty specific in expressing yourself.
Person purchasing the 'other stuff' never knows/cares.
Amazon is happy.
Affiliate is happy.
Who's hurt/injured?
It's a stretch, but you could argue that prices are slightly higher because Amazon has to pay the affiliates.
To some people, whenever anyone is making a profit, however small ... some people believe that someone, somewhere, has to be getting the raw end of the deal.
Note: I'm not saying putting Amazon links on your blog means you can be still be 100% objective (you can't), but rather pointing out a deep-seated belief that I have noticed among some, especially those who've found a way to make a living on GPL'd code, for example, and think iPads should be free.
Also, seriously, his cut doesn't get added to your bill. Almost nothing happens totally for free, and I'd rather have free-to-view content with affiliate links than buy my way through a paywall for every damn thing on the internet.