Tribunal? What does history have to do with it? Fear of abuse!
Why is the amount of games you have played a factor in your punishment? Seem like you would expect poor gamesmanship to decrease with experience. I think Riot has the expectation of its loyal base growing weary o f playing for victory. In the threads I have read, there seems to be a bigger fear of having a system of punishment that will be abused by players over the fear of the punishments against those that deserve it.
Are there a bunch of people abusing the power of a citizen's arrest? You bring up punishing trolls and someone will knee jerk the straw-man of those who break the meta or are unskilled. You know what a goddamn troll is and it's not someone playing Teemo. It's the non-communicator that is so clueless as to playing a team based game that you think he is a bot. Its the guy that leaves 3-5 of his has last 15 games. How bout the guy that starts the game by saying "he is so high right now" and proceeds to feed 20 deaths in 20 minutes. What does the previous 400 games I have played have to do with the fact that I have turned the corner and started to troll my last 10 games?