The updated instructions to developers say: “Please do not include user testimonials in your app description. They tend to be dubious and are frequently utilized to include references to popular search terms and competitor apps in violation of the policies outlined here. Let your users speak for themselves via Play's comment review system.”
Any kind of keyword manipulation to game the search engine has long been frowned upon. Google says it will kick apps out of the Play Store if developers doesn’t abide by the new rules.
Why this matters: Google's been busy spiffing up the Play Store, hoping happier users will buy more, of course. Whatever the motivation, no one will miss those gushy testimonials, especially if they're also manipulative.