The most unfair punishment
Riot doesn't remove punishments that are placed correctly. Hell, sometimes incompetent support does not remove punishments that are placed incorrectly.
People often mistake "correctly" for "fairly". It doesn't work that way. There is a post somewhere nearby with an annoyed guy complaining that he had an Olaf, who didn't change his behaviour after the permaban. For most people permaban is a big shock, which is probably one of the reasons Riot loves it.
Some people quit the game, some people reform and play on a different account. Many won't do either, because the punishment doesn't fit what they did. No matter how hateful and abusive they were, permaban would always feel too much.
Permabanned players will not feel like they are the bad guy, they feel abused and offended. They feel like they are victims.
There are many reasons not to practice permabans for chat and actively look for alternatives, but this might be the main one - it always sends the wrong message, even when the punishment itself might be deserved.