Ever heard of "word of mouth"?
Your average business, or even a very customer-centered business is going to get some unfavorable word-of-mouth, because a customer happened to be there when service was not at its peak, or because for reasons good or bad the customer doesn't like the owner or his family, or because the customer in question is just overly demanding. And you can have spreading untruths about a business, in order to sabotage it. It's the same thing on Yelp, except that the word of mouth is available in text form.
There is of course the threat of inserting false negative or positive reviews into Yelp in return for or to extort payment. I can't get behind that either way, and both are risks in both Yelp and in word of mouth.
However, I get concerned when a business threatens Yelp reviewers with litigation. In this case, the business claims that the negative reviewers are not actual customers, and instead are acting maliciously. Except that this business obviously doesn't know who the reviewers are, otherwise they would not be asking for the identities from Yelp. So I have a serious problem with the plaintiff's reasoning, even if they have a suspicion of who the reviewers are and are looking for confirmation from Yelp.
I wish the judge would have thrown the case out on the grounds of free speech.