Correct me if I'm wrong but one can't do that [charging/licensing by device] in the google play store? isnt that the trade off? google gives a Dev acess the a large potential customer base but in turn the Apps are linked to user accounts not devices.
I am fairly sure a scummy developer CAN do it per device through the dreadful "IAP" concept (In-App-Purchase), which is perfectly allowable in the Google Play environment.