Trolls getting free wins in games where they AFK

So this is my first post on the forums, I had no real incentive to speak my mind as the opinions of low silver players are generally not taken seriously. I just played a game a few minutes ago with a summoner by the name of <REMOVED BY MODERATION> who was by far the most toxic player I've ever had on my team. He banned my pick in champ select claiming to have played with me the day before and saying I was too bad with that champion to play. I let it go because it wasn't really important and I try not to react to trolls. He proceeded to get a huge gold lead on the enemy by stealing all of my redside camps, following me around and making me smite things to get the gold. After his first death at 12 ish minutes he disconnected and reconnected every 10 or so minutes, afking in base. We ended up winning anyways thanks to a good baron call and out of curiosity I checked his opgg to see if he gained LP from the match and to my surprise he did. There was no sign whatsoever that he had afked for the vast majority of the game. So I guess my question is this: why should he benefit from my and my teammates hard work when he did everything he could to frustrate us and didn't even play most of the game?