Google has removed Amazon's app via search from its Google Play listings.
Back in September, Google changed its policies, requiring Amazon to replace its app within its Android store. Inside the new app was a functional app store, which as a competitor, Google didn't allow users to download. Amazon never publicly announced the hidden app store, something that was initially noticed by Caschys Blog, a German technology news website.
At the time, Google's developer agreement barred "[using] the Market to distribute or make available any Product whose primary purpose is to facilitate the distribution of software applications and games for use on Android devices outside of the Market." It has since been changed, to ban any product with any purpose to do so, whether primary or not.
Amazon's app store is no longer available within Google's, its removal as quiet as its launch. Though Amazon's app still has an active URL, it's no longer searchable within Google Play.