Actually, depending on the content of the article and the work-relationship with the paper, the writer of an article can be sued. Also, if you're writing for a newspaper that has any sort of real coverage (not some local market shopper or low circulation paper), then a lot of those articles... especially the ones that might jump-start a lawsuit are vetted, edited and checked multiple times.
Mind you, if they're just repeating something off of the AP wire, then that tends to pass the buck back up the food chain.
Letters to the editor skip past this with the "The views expressed in these letters are not the views of this paper" boilerplate.
As for why these reviewers could get sued. Well, it's the U.S. You can initiate a lawsuit against practically anyone for practically any reason. Actually winning the lawsuit can be a lot trickier, though.
Also, a lot of these review sites, especially ones like Yelp, typically contain some legal boilerplate saying that the reviewer is the one responsible for the content of their review, you have to be honest about it, etc.
So, if these are fake reviews (the reviewer never actually used the business in question), and are just badmouthing the business to be jerks... well, that's actionable.