The risk (mostly) isn't fake email screenshots, the risk is that I create postings for (or even commit to paying for fake reviews for) my competitor, and tarnish their reputation. It's hard to establish provenance of such an arrangement.
But from Yelp's perspective, a paid-for review claiming bad food is just fine, because it only tarnishes the restaurant's reputation. On the other hand, a claim of fraudulent review manipulation hits both the business and Yelp's reputations.
A review claiming paid-for-fraud isn't so easy to wave off.