Give you back lp when someone becomes toxic/trolls and gets banned

I had a troll in 1 of my games, reported him and he got an instant feedback report in like 2 minutes. The problem is that, why would Riot not give compensation when you have a troll/inting person in your game.

Like they knows it hard enough facing trolls/toxic kids who are flaming you and running it down. If they get banned, they did something wrong right? Then why wouldn't they just give you back the lp if you lose a game?