From the perspective of a large advertising firm, consistently replicating that at scale gives you a lot more inventory to sell.
One could say something similar regarding content farms and big daddy G.
They're afraid of competitors leaving negative reviews for a business and there's probably something to that, but there's no transparency about it. And they've been doing the extortion thing for a while now: http://dealbook.nytimes.com/2010/03/19/yelp-under-fire-from-...
I always thought it would be great if there was a Yelp for landlords - because landlords are rarely held accountable and it's not like you ask for references before you sign a lease - but there would probably be too many false reviews posted to make it work or tenants wouldn't feel comfortable speaking freely (because unfortunately tenants still need references). There's actually a website trying it, but doesn't seem like it's going anywhere right now: http://www.donotrent.com
Hopefully people will see Yelp for what it is: a small sample of the population that may be dissimilar to you writing probably biased reviews.