<<<….”They are hired by businesses a) to monitor all online reviews pertaining to them and b) to get rid of “bad reveiws” on, e.g., Google, Yelp, Yahoo, etc.”….>>>

How do these internet reputation preservation/restoration companies “get rid of” bad reviews?  They can’t really send over Moose, Rocco, and Vinny with some baseball bats to the Google and Yelp headquarters. 
Glenn

I believe they send lawyers threatening suit for publishing libel/slander in interference with commerce.
 
Many who post negative reviews for non-objective reasons or who have “hidden agendas” simply cannot resist “going over the top”.  This makes it easy for the reputation defender firm to convince a site, e.g., Google or Urbanspoon, to pull the offending post.