I believe there are some instances where business owners claim that Yelp offered to 'make bad reviews go away' if they sign up the premium service (i.e. start paying Yelp money). Even if Yelp! doesn't mess with the reviews otherwise (flagging positive reviews, creating bogus negative reviews, etc), that seems very much anti-user (at least towards the people looking for recommendations).

Yeah, but homeopathy has no financial incentive to work. Chemistry just doesn't care about things like revenue. Yelp! on the other hand...

It is also a common belief by some people that breathing air is necessary to stay alive