Don't think Roku boxes have standby circuitry, spare for Roku 4. And btw, it's the use of IR remote that allows RokuTV to turn itself "completely" off instead of idling to listen on wifi.
Whether they can completely cut the video signal is sort of a moot point. Why not simply offer an all-black screensaver and allow the TV to make a *Smart* decision?
It's not as though a black & white screen with a rather static Roku logo showcases the best of the platform. And if they're adamant about the free advertising, then limiting it to 15-30 minutes would be great on my energy bill.