There are news stories popping up all over the country of Businesses suing people who have left negative Yelp Reviews. The first thing that comes to people's minds is What About Free Speech? What About Our Basic Rights?
The short answer to the Yelp Lawsuits is simple: Defamation and Misrepresentation. Theoretically speaking, if someone leaves a negative review and it's not true it can fall under Defamation or Misrepresentation lawsuits (depending on your state laws). So from the perspective of a reviewer (or consumer), you should be careful when leaving a review to ensure that everything in there is completely true. Do not open yourself up to liability!
To the employers who are engaging in these lawsuits. Unless the review is so blatantly false and damning that you must file a lawsuit... there has to be a better way to engage your customers on Yelp to get the reviews fixed? I would recommend contacting the customer privately, trying to speak with them on the telephone, and trying to understand their side of things before leaping on the offensive attack.