This person wrote a blog post with a few Amazon links under his affiliate code. Some people read his blog post, click the links, (and probably read the reviews), and decided to buy the book.
The person gets the kickback, the interested readers get the books they want to read, and everybody is happy.
But why am I reading stuff like "getting away with murder", "purity", and etc?
Why such negativity/hate?
A concrete example would be writing an especially glowing review of the new Kindle because you have a vested stake in people buying them. Or, perhaps, NOT writing a glowing review because you fear it will be perceived as shilling for affiliate cash.
It's the reason why most reputable newspapers do not allow their ad sales staff to communicate with the editorial staff about content.
In fact On The Media recently did a story about the Washington Post struggling with whether or not to include Amazon affiliate links in its book reviews. I think it presents both sides of the argument: http://www.onthemedia.org/2011/nov/11/web-links-money-makers...
I'd like to think that a regular reader of a publication should be able to figure out if that publication is shilling when it should be objective. On my Nine Inch Nails site, when I link to Amazon, it's about a CD/Vinyl/MP3 album that just came out, and I think that makes sense. If I posted about some shitty Trent Reznor bio and mentioned the linkKindle Version/link available for the linkKindle/link I fully expect my readers to flog me. And I'm not saying that every blog out there pays attention to editorial ethics the way I feel I do, and I've given people a hard time about sneaky affiliate links too. But I think that there is a 'right' way to use them.
When I started my site a dozen years ago, I wanted it to be a place people went to read NIN news. Money wasn't part of the equation. But if you're already linking to sites that have affiliate programs, you'd be foolish not to take advantage of those affiliate programs.
If someone writes a meaningful glowing review of the new Kindle, then there should be information and details in the content of the writing that explains why they think it is so great. If there is no such information in the article, then the article probably isn't very helpful, and neither the presence nor absence of affiliate links will change that.
But what if it is an awful fluffy review written for the express purpose of making an affiliate sale? Is this wrong, per se? If something in the article either directly or indirectly prompts a reader to click on the link and they make the purchase, then what harm has been done?
Newspapers likely disallow such practices in order to maintain journalistic integrity, but a blog author who is writing posts on purpose to sell things is probably not interested in maintaining journalistic integrity. The blog author is just interested in selling stuff. Maybe the blog posts are well-written and interesting, or maybe they are not. If they are not, then readers who care principally about content will likely avoid the blog on the lack of merit of the content itself.
A product review is close to useless if you have no trust in its author. The crux of a review is the author's opinion about something. Whatever verifiable facts are contained in the review are still only those facts the author chooses to highlight. Consider: if authorship doesn't matter, why is a review different from a press release? In short, a review cannot speak for itself because context matters.
Let's set aside the ambiguous case of adding affiliate links and talk about out-and-out payola. If a blogger takes a secret cash payment to write a false product review, then I believe that is dishonest and wrong no matter how well written or useful the review may be. (And not to confuse what's legal with what's ethical, but taking cash for reviews on your blog might even be illegal[1].)
[1] http://www.cjr.org/the_kicker/ftc_votes_to_end_blogger_payol...
You might be in a position to turn down free money for hypothetical reasons, but I'm not.
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?
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.
It's a stretch, but you could argue that prices are slightly higher because Amazon has to pay the affiliates.
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.