When there's a string of bad reviews on Yelp, it's basically Yelp's way of telling the shop owner: "Gee, doesn't it suck that these spammers are bad mouthing your business without actually patronizing it? Well, if you work with us I'm sure we can weed out the spammers and only keep the honest (read: positive) reviews."

In this case, it's: "Gee, it's regrettable that our Zestimate for your house is so low. Only if we had a better source of data. Well, if you work with us I'm sure we can get a better (read: higher) estimate that's more in line with the true value of your house."

The buyers will flock to whichever market maker that advertises the lowest price, in this case Zillow. This in turn forces the seller to choose Zillow as well since that's where most of the buyers are.