While there are definitely anecdotes about this kind of behavior, I have yet to see evidence of systemic or corporate-sponsored effort to do this. I believe this is more a question of possibly poorly-incentivized sales people rather than an intentional strategy on behalf of Yelp. I think this is backed up by a number of lawsuits that haven't gone anywhere. I might be mistaken; if there is a source showing this is really systemic, I'd be very interested.
Either way, Google now does many of the same things that Yelp does (reviews, photos, metadata management) - but it doesn't really care about the space, apart from being good enough to beat its competition. While today maybe they're motivated to build better products to compete with Yelp for eyeballs, overtime I think this means the reviews space will stagnate. That's unfortunate.
I think this is the core Google problem: They have so much top-of-funnel traffic that they just have to do things "good enough" to crush entire industries of competition. The bar to be sufficiently better enough than "good enough" to build a meaningful competitor to G in any vertical is so high it stops many potential competitors from even trying. The only companies that manage it are ones that have entirely different & massive profitable business models like Apple.
Everywhere Google is, the sector stagnates: Search! RSS readers (with a bit of a renaissance since Google Reader died). Email. Calendars. News lists. Increasingly, reviews. Fortunately MSFT pushes them in docs and hopefully Apple continues to do so in Maps and browsers.