Thank you for posting this in the answers forum! I'm sorry to hear you are unable to power on and off your cable box via Slingbox. The cable box does need to be powered on, in order to get a video signal. The on-screen remote should be allowing you to power on/off your cable box. If it is not, the issue is most likely the on-screen remote not functioning. Who is your cable provider?
Feel free to reply!
The Sling Moderation Team