Anecdotal but I did reputation cleanup for a company. I never recalled any 'we'll remove low star reviews' offers from the Yelp sales team. My guess is some sales guys get desperate and the Yelp ad package was not cheap/scaleable like Google ads so they definitely had to dig.

Incidentally removing low star reviews from some sites is easy(prove review violates TOS) .... I remember Yelp being strict on not doing so.

If you want to see some real grade A con games....look up Groupon. Great idea, greedy as hell execution.