Dungeon leaving ban?

So we get into the dungeon and start running, tank then leaves right away, me another DPS and healer fight a few groups and I'm not feeling like doing a 3 man dungeon, I look at what it says to leave and it says a penalty will incur if I'm the first to leave the dungeon or the group, well I wasn't first for anything the tank was already long gone, so why do I still get the 30 minute ban