I have been a customer of Newegg for years, 4 to be exact. I always read the reviews and never stop there in my hunt for info. I have read thousands, perhaps 10's of thousands of Newegg customer reviews. What is generally found is that Newegg has the best info all in one place and of course this is in general terms.
With that said, there are thousands and thousands of negative reviews on Newegg and plenty of positive reviews that actually provide any useful info. It is extremely doubtful that they do not allow any negative reviews to be posted.
In regards to them not posting any negative reviews, it seems they simply do their best to remove misleading reviews, either positive or negative. Like was mentioned in this thread, one reviewer claims the fan is loud and the other reviewer claims it is quiet, this is the nature of subjective opinion. The same example could be applied to defective items. One customer could receive a defective item and think the product is terrible and the manufacturer is terrible whereas the next customer to post a review did not receive a defective item and they believe it is the best product in the world and the manufacturer is the top in the industry.
One reason I believe there is a large quantity of positive reviews versus negative reviews is due to the fact that a majority of customers that get working products and the products meet at least what they were expecting are happy. Some negatives seem to rarely offer any reason for the dislike, simply that it is a terrible product, or it is a terrible product because the luck of the draw defective came their way. Manufacturers make little or no money and neither due vendors or resellers like Newegg, especially Newegg on selling items that are primarily defective or terrible as an inherent quality. So even if there were items that were inherently defective or simply terrible products and collect a substantial quantity of negative reviews they would not last long on the site due to the substantial losses incurred; by the customers (customers become upset), by the reseller (good resellers take care of their customers), by the manufacturer (manufacturers must be good to their resellers), ect... Everyone loses when it comes to defective products or very poor quality products, in general terms of course, the shipping companies make out very well. When the products are removed from the site, so would all the reviews for those items and it would also lend to the appearance that only positive reviews were posted.
Considering how Newegg treats their customers and the exceptionally flexible services they provide, I doubt there would be any benefit for them to encourage the sales of inherently terrible or defective products or simply decline all negative reviews.