Read the Article.
Then, put yourself in TripAdvisor's (or any online review publisher's) shoes. If you publish something that is libelous you could be sued. If you publish allegations that damage a business, and those allegations turn out to be untrue, you could be sued.
I'm not necessarily defending TripAdvisor's business practices, because, as the article said, they aren't disclosing much about those business practices. But I am pointing out that a business like TripAdvisor HAS TO protect itself somehow from publishing stuff that is untrue and that could damage a business.
Not that any internet user ever puts anything untrue out there. No.
-Speak