In addition, tribunal punishments are decided way after games are finished. So there would be no way to give you back the LP you lost from a game.
Here is an example. You lose a game in S1 because of someone. Everyone on the team reports him (because he was legitimately doing something very wrong). You play for the next 2 days, have some really good teammates (and lets be honest, luck) and you make it to gold IV. Now the punishment of that guy from your S1 game comes through. Do you realize deserve LP (and a chance to get further up) and a division where you didn't win a game? My answer to that is no.
Even more complicated: What happens if you are in a series and the player from the S1 game gets punished? There is no way to give you back those points. Its annoying, but that is just the nature of playing a game online with people.
And then there is the abuse case people are talking about. And while they would be able to tell a meaningless report from an actual one (so no real abuse would happen) it would clog up the works for who/what ever sorts through reports. So not worth it.