STB stays on for a variety of issues:
1) If it is a DVR, it has to stay on to be able to record programs.
2) All STBs have to stay on to get routine guide updates.
3) All STBs have to stay on to get software updates.
For 2&3, if they powered down (like you do) there is a delay when you want to watch TV that most customers would find unacceptable.
The newest STBs are energy start compliant.
BTW, turning your TV off doesn't reduce its power usage to 0 either.
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