Why does leaverbuster punish you when playing co op vs ai?

Last year, I would always play a game of bots before going to school so that the game would end quick enough. Sometimes, it didnt end quick enough, and I would have to leave the game. Unfortunately, I got banned very frequently from this, despite it being against beginner bots. This still happens today, so why does it punish you? it's not like it will make a huge difference on your team's performance, because the bots are easy. I have actually won a beginner bots game as olaf when my entire team dc'd. It is very easy on purpose.