It is weird. They do turn it off sometimes. I've been to DC thrice in my life. They keep it lit for multiple reasons the main reason being that it helps orient pilots, police and tourists. If you see it then you know where the center of the city is. It also provides light for tourists that are there past sundown. If it's off something's up. Could be that the bulbs burnt out all the way up to they're moving something big into the area.