The "average review site" is a rather wide net to cast, you'll find a lot of them are setup purely as affiliate marketing tools - benefiting the owner by using carefully placed referral links. It's in the owners interest to minimize the amount of bad reviews, because they get paid when people sign up and start spending money. You're right about fake reviews, both because certain ones might be made up simply to produce content and others can just be from disgruntled customers who never understood the odds they were playing against. This last type of customer is likely to be very angry at losing money and leaves a malicious review, even though they should have known better all along.