I've heard horror stories of hotels who are blackmailed by guests into getting a discount or otherwise they will receive a bad review, and those who get bad reviews find that it does impact their business. No great shakes if you're the Hilton maybe, but for a small business owner it can be devastating.
Fake online reviews are not particularly hard to spot. When they're glowing, full of marketing speak, and from a new commenter, this should raise a flag for website owners and users. On the other hand, some hotels and restaurants have complained about malicious and unfounded reviews, some of which come from rival companies.