Leaver Buster in Co-Op Vs AI

I got up for what was supposed to be a few seconds to check on something during an AI match. My mother saw me, told me to get something, so I decided, why not. I was gone for maybe 5 minutes because it was, well, a bot match and my team was, of course, dominating. I come back to it saying I was afk for too long, and it kicked me. No problem, I reconnected and rejoined, telling them sorry for the AFK, though as soon as I said it, the Rek'Sai got a triple, so I naturally jokingly continued "Not like it matters". After the match, I get a Leaver Buster warning. Why is Leaver Buster counted for Co-Op Vs AI? Now I understand that it can have players not think leaving is fine, but for someone that has been playing for years, and is Lv30 I doubt I need to be reminded about this. It is a good system for Ranked and such, but for AI? I've lost 2, and that was when I had FIRST started in Season 1. Now I'm penalized for "[making] an unfair game for [my] teammates" in a bot match.