The “Dangerous Products” section of the Google Play developer policies now reads, “An app downloaded from Google Play may not modify, replace or update its own APK binary code using any method other than Google Play’s update mechanism.”
The update apparently came on Thursday and is seemingly a move to curb update mechanism as used by Facebook a couple of weeks ago whereby users were not required to navigate to Google Play app store to apply the update. The update will help Google eliminate those apps which otherwise would have bypassed Play store’s update mechanism to update themselves with malware.
Android has a reputation of being far more lenient than iOS as it allows installation of third party applications if the user explicitly permits it. But, it is highly unusual for apps downloaded through Google Play Store to utilize an update mechanism other than that of the Play Store. The update in the policies would force companies to use legal update mechanism and do away with direct update mechanism.