buttons light up all the time.
Could be an app or glitch causing this.
how long have you had the phone?
has it been this way since day one?
are you rooted?
install any apps that can take over settings for screen, themes, etc?
did you do a restart on the shutdown menu to see if this corrected the situation?
edit: WITH auto on, screen brightness and the buttons will dim according to the ambient light. so when you are in extremely bright areas and the sensor adjusts, the buttons will go dim and the screen will increase in intensity.
on average though, the buttons will be lit in normal use (unless you are around extremely strong light sources)