At the end of the day I support Amazon's decision on this.
If they were supposed to police everything people posted on their site(s) for veracity, then I'd guess the vast majority of reviews (positive and negative) would have to be removed. Many companies have engaged in posting phony positive reviews about their products and phony negative reviews about the competition. Yes, that's a job that you can get.
Hence the reason why a number of places now have "verified owner" reviews which have a higher impact on the rating than unverified owner reviews. Not to prevent bad mouthing, rather, just to prevent *competitors* from getting in on the game.
Quite frankly, the upvote/downvote activity on this topic is very interesting. So much so that I'd hazard a guess that various people who work for the company in question are down voting anything that doesn't berate Amazon.