How exactly are players that give up every game still allowed to play?

Just had a Thresh that got autofilled support, died twice in four minutes, then said "bye" and went AFK for the rest of the game. After seeing this, our ADC, Caitlyn, declared that the game was over and she was going to troll. She then proceeded to soft int by continuing to go bot lane for the entire game, even when the enemy team was pushing our inhib. Riot, do you mind explaining to me exactly why someone intentionally ruining games for other players because they are performing badly are still allowed to play whenever they want with no fear of punishment? While players that lash out at the griefers can easily receive chat restrictions or even bans for swearing at them or calling them the dreaded R-word that, even when censored, is removed from the boards? Toxicity is not the only problem this game has, wake up and start trying harder to detect these people.