Why did I lose 23 LP because our last pick decided to troll?

It said it wanted AD, we gave it AD, it instantly locked in Kayle. It then proceeded to feed a bit, and stopped playing. It just kept walking back and forth from lane to the fountain, and occasionally ulted someone. Meanwhile, it's constantly typing as if it's not trolling, presumably to avoid being banned. After the game, it reported the entire team for verbal abuse, including the enemy team.

This is pretty fucking r%%%%%ed.