Why should I receive a penalty for exiting a game I've already won?

So get this, I'm kog'maw and we push down nexus and win the game. Problem is, my victory screen where I can click to go to post game doesn't come up. None of us can move. So when I press esc and click exit game it says I'm going to receive a penalty for leaving a game. What gives Riot? I've already won. Why should I receive a penalty in this case for my victory screen not showing up?