When a case is finished, if it has a clear majority or either Punish or Pardon, it is lumped into that category. Those who voted with the majority would be considered "right" in their voting. Therefore, it is assumed that more people will vote correctly than not, and if you vote with that majority (which should be correct) then you are correct too and are rewarded.
As for the case you bring up, it has nothing to do with the Tribunal being broken. That report would have gotten made either way, the difference is that instead of a Riot guy looking through every report, we have the chance to decide if it is a valid complaint or not, and if it is, then Riot can look at it and decide if that Trundle should be punished. If he reported Anivia for the trolling, she will come up in Tribunal also, but you didn't receive that case. If you believe that the report on Trundle was false/incorrect, you are there to Pardon it and make sure that guy doesn't get in trouble. That is the point of the Tribunal, and I don't see how it is broken.
And the Tribunal is broken because most people will see what Trundle did as wrong because he used "curse" words. That's all that showed in the chats, him freaking out, but what you don't see is the pathetic in-game actions of this Anivia.