Clearly HBO is walking a fine line here: The cable companies that partially subsidize them in order to lure subscribers don't want you walking over to your (non-subscriber) friend's house and throwing "Game of Thrones" up on the big screen via Apple TV.
The whole "let me pay for it!" debate has been well-covered elsewhere: In short, HBO probably couldn't make a living charging individual subscribers for content, especially on a per-show basis. How could they produce new material without the guarantee of N-million monthly subscribers? And cable/fiber/satellite companies who help promote their service?