Here's an example of where Yelp specifically went after companies who compensated people for reviews: https://searchenginewatch.com/sew/news/2323483/yelp-ceo-confirms-sting-operations-to-clean-up-paid-reviews
The other angle is the policies of online search engines and business directories. Google, Facebook, Yelp and others all make it very clear that they do not tolerate consumer reviews that are compensated. Part of this is that THEY could get in trouble with the FTC, but the other part is that they want their own users to have a safe, trusted experience.
