The problem is that spamming punish generally will result in a "good" justice rating on the Tribunal.
The reason for this is that much of the LoL community is composed of crybaby whiny kids that probably use the phrase "I'm telling the teacher!" regularly in a typical day of their lives.
The percentages are relevant in the Tribunal judge demographics as well: most people judging the cases are the same way. Most people, for that matter, who "report" people are like this.
Cases where someone simply makes some offhand comment that might be loosely deemed "mean" or "offensive" regularly result in negative Tribunal votes (i.e., votes for "Punish".)
TL;DR: If you offer rewards for the Tribunal, people will just log in and spam Punish. Because of the kiddie-kid whiny-baby nature of the LoL-community at large, this will actually give them a positive rating and net them rewards... resulting in a snowball effect where more bans are handed out, more people are reported for silly things, etc., and on and on... and on.