Why would this not be the subject of a defamation suit?
Are review sites normally the target of such suits or is it the poster of the review? These would be linking to credible news outlets, so seems like the owner should take it up with them. My guess is it is hard to win a defamation suit against the lawyers for someone like the NYT.
"Don't blame us! A source with no proof and an axe to grind, told unverifiable gossip to an unaccountable journalist, who just happens to agree with us politically. But we're credible, so you're a racist now."
This is actually a good thing. Though the mainstream media has very little remaining credibility, Yelp will help eradicate the remaining shreds by further exposing the myth that media has built around itself; namely that they are unbiased, trustworthy purveyors of truth.