The major problem with that, is that would leave it up to players to decide whether someone is trolling or not. No matter their state of mind, they get to decide that someone is trolling, whether its true or not. Too abuseable, and too unrealistic if you expect Riot to go back and redeem LP after they've confirmed whether or not there was a troll. Thats like double the work for something that is already too much work.
As far as champ select, theres VERY little to indicate someone is going to troll. I pick anivia bot a lot. I get called a troll sometimes. My response to those assannine accusations, is pure sarcasm. "Yeah dude im totally a troll". I have no fear because Im not trolling. If theyre gonna be assholes about MY pick in a video game, im gonna be an asshole back because I dont give a fuck what they think. Riot will review it and find I wasnt trolling, and Ill be fine, while their imaginations will ruin their own experience.