Probably from the ability to play any of your games anywhere, anytime. Visit a friend, sign in your gamertag and download one of your DD games to play. You sign out and go home, the DD is still sitting on that console.
24 check prevents it from being used on your friends gamertag.
My best guess.....
Well, a disc gets you DD rights so maybe the system does not differentiate between the two yet.
Which brings up a good idea. DD purchase on main console allows for offline play. If you login at another location it checks your library based on your DD purchases and lets you play (online). Negative is one license could be played at same time.