As some others have said, the tailights don't come on when the automatic headlights are on. You have to turn the regular headlights on for your car to be seen from behind.
It is the law here, if you need wipers turn on headlights. I think some people turn off their auto lights to be funny or cute. A grey or silver vehicle is very hard to see at dawn or dusk when it is foggy. My last two trucks were Silver, my 2012 is White Diamond and is much easier to see. I turn on my headlights when it rains, not much this summer, not much rain here, also when it is foggy or just real cloudy. I use my turn signals, even if noone is around.