Any developer can require a monthly service charge to keep on "logging in" so to speak. If you buy a game that requires multi-player over the net, they can easily require a monthly charge for the app to connet to their server and keep playing. This is like World of Warcraft and other games do. There may be an app that has built in update code that requires a montly fee or some other form or registration to get the updates.. or updated data that the app uses, etc. Plenty of ways to extract money on a monthly/subscription base if need be.