LOSING LP FOR A GAME WHERE I AFKED BUT CAME BACK AND WON

So i played this katarina game and it seemed laning phase started my computer disconnected from the WiFi automatically. I then came back, apologized to my team, and we won the game. The problem was, i lost 18 lp for it when we won? I know it was my fault for afking, but why did i lose lp for it? I might as well have not reconnected to the game and let my team have a bigger chance of losing if i knew i was going to lose 18 lp for leaving?