Apparently this is only available on the Roku 4. Why? No one knows.
Why do I need this? Because I often fall asleep with my TV on and it runs all night. My TV has auto-off, but since Roku is always-on my TV won't turn off. No reason why this shouldn't be a option on ALL Roku devices. I don't need to have my TV run all night because Roku does not turn off, even though there is no activity.