True, but realistically the time involved with moderating every single review would be astronomic. I find that in the long run the bad reviews don't really matter, and that goes for any book. Just skimming the reviews reveals the authors are simply pushing their own agendas. Their "opinion" are easily dismissed and are usually firmly challenged in the comments. If you come here with your bias firmly cemented in place, you might be deterred from buying the book. But if that's the case, you probably would have tossed the book aside pretty quickly anyway. But the "deniers" who just regurgitate their own biases eventually get drowned out by the vast majority who actually bother to read the book. If the book is bad, that won't help. But if the book is exceptional, like GCBC, the attempt to derail the book fails miserably.
I've seen this happen for quite a few books. The most recent example is the book Atkins for a New You. Campbell posted a long irrelevant negative review and got all of his zombies to post some as well. (Or most likely someone else urged them to go 'support' their guru). Which was a mistake because it makes Campbell attack on the book even more transparent. In the end it doesn't matter. The good reviews far outweigh the bad, and the book still has 4 1/2 stars.