It's working as intended. The Tribunal is to punish thoughs that breakt he Summoners Code, i.e. ruining the game for others on purpose, through verbal abuse, trolling, feeding, so forth.
A good on is ARAM (All Random All Mid). That can be incredibly fun to play, and can really take some time to get through since leveling is so slow and most play cautiously. The rules are known, and if they are not, they are told usually at the start of every game. But if the most basic rules are easy to follow, don't go down the other lanes and push or farm in the jungle, and don't just run into the team repeatedly to feed. If someone starts doing that mid-way through they just ruined a good time for 9 others that enjoyed that first 25--30 minutes and is now no fun. If a Troll joins someones custom game, he shouldn't feel safe to troll and ruin the whole game experince because he didn't do well and was feeling cheeky now.
© 2013 Riot Games, Inc. All rights reserved. Riot Games, League of Legends and PvP.net are trademarks, services marks, or registered trademarks of Riot Games, Inc.