I believe individuals have the ethical responsibility of telling the truth about local businesses they had an experience with whether it was good or bad. Having a bad experience and then stretching the truth about it on YELP, or any other site of this type, is no different than having a good experience and doing the same. The public lost it’s ethical responsibility, as far as I’m concerned, as soon as social media and basically the internet came around.
I don’t think these sites should offer advertisements of local businesses because that gives your company somewhat of a liability. If a certain business is paying for advertisement on your site, and a rival business figures that out, then they could use that as a reason that they are getting bad reviews, in which they might file suit against you.
I do believe that sites of this sort need both positive and negative review’s to promote authenticity. If the only thing they had were good reviews, people wouldn’t think the site was credible. If the only reviews it had were negative, then businesses would consider them basically a hate group.