Thanks for the warning. To be fair, even big "companies" like yelp.com have been running extortion rackets for years and are still the darlings of the technical press.
http://www.wired.com/threatlevel/2010/02/yelp-sued-for-alleg...
I don't know much about yelp, and don't use it much, but have generally been led to believe that they are a good company (admittedly, by the tech press) so it'll be interesting to hear if you have actual examples of people getting scammed.