LeaverBuster doesn't punish for leaving/afking just one game, it punishes for consistently leaving/afking multiple games over a short period of time. From Ulanopo's Knowledge Base:
I only left one game and it was for a good reason. I shouldnt have been banned.
Two quick things:
[1] No one in this forum can help you with a LeaverBuster ban. Thats a separate system and we have no oversight or control over it.
[2] A LeaverBuster ban occurs because you have accumulated a large number of leaves and/or AFK time. It is your responsibility to fix your **** before it reaches that level.
That established, there is NO good reason to leave a match. The system is tuned so that you can leave if you have to, but only on an infrequent basis. Its all the leaves and AFKs before the one that triggered LeaverBuster that led to your ban. Its like using up all your sick leave to go to the beach, then complaining that you dont have any PTO when you fall ill. YOU chose to use up your leeway with the system.
What the other person did doesn't matter as this is about you and you alone. If the other person has a habit of intentionally feeding then they will eventually be punished. Again though, this case is about you and you alone.
P.S. Having a civil discussion means not calling or assuming that anyone who disagrees with you is a Riot fanboy. If you want Yesman answers then you're out of luck.