Facebook had been confusing users with unsolicited 'beta' updates outside the app store
Google recently changed its Google Play Developer Program Policies to prevent applications from using Google's app store and then going off the reservation for updates.
The change likely is in direct response to Facebook doing that very thing under the guise of a "beta" program. While there might be any malicious intent there, it is a bit specious, and confusing to consumers.
The new policy language reads thusly:
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.
That should still leave room for applications that are distributed through Google Play to simultaneously have beta tracks outside of Google's app store, so long as those trains stay on their proper tracks.