Wait, why on earth should people be prevented from leaving a negative review when they have a negative experience? A review site that doesn't let people leave negative reviews is an untrustworthy review site.

Sure, people are unreasonable - but that has nothing to do with Yelp! Think of it this way: you can make it look like the world experienced vastly more earthquakes in the 20th century than in the 17th, but what actually happened was that detection got much better. Similarly, Yelp doesn't at all change people's propensity to think unreasonable things about restaurants they go to: it just improves detection. Improved detection, I argue, is an inherent good. Knowing what people really want makes it easier to deliver that, even if the process of finding out what people actually want is painful (and it usually is, since most of us are not very good at knowing what we actually want).