I do lots of web work for my clients, and many of them simply remove negative postings or add their own positive reviews.
This isn't limited to simply product reviews - there are big companies out there that set up programs to increase site hits to artificially increase sales revenue, set up massive twitter & facebook campaigns to create "buzz", and the list goes on and on. Not to say Nashbar does this. But think about it - they are in the business of selling product. If there's 10 negative reviews posted about something, they wouldn't sell product. Period. Really, the only way to get an accurate review of something is to ask people you know or go to an independent site. And even then, there are representatives of companies that are in business to post positive things posing as a regular Joe. Or the site can be paid off by a company discouraging negative posts.
Again, my experience w/ Nashbar (and pb) has been positive, but it doesn't make business sense for them to keep negative posts. It does make sense to keep 1 "negative" one and 4-5 positive ones because if someone wants a product, they will "side" with the positive reviewers. This presents a fake "fair and balanced" view of their products, but one that really works to sell online.