Goto Setting->General->Sleep-After and change the sleep time, the light wont go out until the ATV sleeps, the default is 1 hour.
I have noticed this too.
When I go to HDMI 1 the Apple TV3 is asleep but the light is on.
Also, I have sold my Apple TV2's, but can anybody who has both say whether the Apple TV3 light is brighter or bigger. I don't remember it being as intrusive on the Apple TV2's.