The piece that was originally here has been moved. (The comments remain attached to the present post.) It was a piece produced for amusement that mined the Amazon reviews for a particular book in search of evidence that Amazon had censored one-star reviews. It turned out that the evidence didn't stand up. The piece originally stated that the evidence might not hold up and pointed out the reason it might not, which did in fact turn out to be the case. The piece got a lot of attention anyway. At this point I'd rather have it elsewhere than on my blog.
However, based on the (limited and anecdotal) evidence I've seen, I *think* Amazon may have guidelines in place that make it harder for negative reviews to get posted than positive ones, even though those guidelines probably aren't "don't post negative reviews." Rather, negative reviews may to have more hoops to jump through. See Brian Powell's experience for more.