[guiltygatorade]

Probably because most users are blissfully ignorant of the less ethical bits of how Yelp conducts its business.

As long as there isn't a huge discrepancy between the expectation and the services/goods rendered, most people can't tell the difference over something that is largely subjective anyway.

[sak5sk]

This and many users are hooked on the social networking effect where other users are liking their reviews. Twitter is much the same - you get a constant stream of endorphins every time you see people liking and retweeting your crap. It's socially toxic but effective.