When digital TV inputs (ie. HDMI) have caught on a bit more, presumably it would be easy enough for the box to detect whether or not the TV is being watched, and if it's not then go into standby. I'm guessing at the moment with analog interfaces like composite and s-video, there's no way for the box to detect whether the TV is on or not so it has to rely on user input to know whether it's in use or not.