Why do you lose LP when dodging a lobby?

It's not really fair to me. You are not in-game yet. Why would you(Riot) take LP from me then? I often see trollers in my team pre-game. For example I search a teammate on op.gg and I see that in the last 10 games he lost everything and has scores like 2/18/1 and 0/20/3. Some people even say in pre-game: i'm going to troll LOL Why would I bother playing when I know pretty sure he is going to troll in my game? Why is that being punished? Should I just go through an painful 20-30-40 min game where he is going to ruin everything, and then lose a bunch of LP, for what? We could've easily tackled that pre-game.
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