It's a pretty expensive undertaking. You have to be a subscriber (at non-trivial cost) to be a member/reviewer. The vendor would have to create multiple puppet accounts at $20/pop to shill up reviews. But, what of all the other actual reviewers that will write bad ones? You tend to see patterns in the reviews and, when a shining review of a vendor leaves me skeptical, I always look at the members' other reviews to see if they left at least 2 or more. If they did, then I suspect less that they're in cahoots with the one vendor.
If you're always double checking reviewers, it's easy to do the same thing on Yelp, or anywhere else for that matter. Most people don't, which is why Yelp filters and Amazon ranks.
The Limbaugh thing is unfortunate, that's true.