If the negative reviews are mostly whiny,emotional rants without any valid solid arguments, then the book is most definitely worth it. A case in point:
http://www.amazon.com/Structure-Interpretation-Computer-Prog...
Somewhat surprisingly, the median review score is 5, and if you look at median scores for products, the skew is very much left. So, the data pretty much backs up your observation!
This works surprisingly well for any product on Amazon.
Another thing that is really annoying is people buying wrong product and leaving angry reviews. Whose mistake is it if I buy a product thinking it is water proof, when the description of the product says in bold letters that it isn't waterproof? I feel sorry for the sellers who get idiotic/unfair reviews and they can do nothing about it
But if you still want to address this problem, just allow the buyer to say it is for someone else and allow them to put the recipient's email. If I buy something for you, I put your email, and Amazon could allow you to review the item instead of me reviewing it.
Steam does something similar in having the humour moderation so that things don't get moderated as useful just because they're funny.