A lot of older cars didn't turn off their OBD port, have their headunits go into standby, or even turn off the cigarette lighter port. Early OBD ports connected to dealer computers for a few minutes, not an always on dongle. Plain headunits just play music, what could they possibly accomplish by staying on when you turn off the car? It was a convenience having the cigarette outlet left powered so you could light a cigarette without turning on the car. Other than maybe a bag phone, what would you possibly plug into that?

If that was one of those ELM327 dongles, yes they have 12V and are known to drain your battery. They're only meant for short diagnostic runs.