it doesn't make sense, banning people permamently for flaming in game is like putting ppl in jail for yelling some weird shit to other people. Imagine that you have a bad day, you want to play a game. It doesn't matter to you if you win or lose just want to play game - instead of that you got a troll guy who don't want play support etc. and go in your lane and troll you. So you write some shit about him and next day you are perma banned from game where you spend a lot of cash. The guy who acctually troll game for others people in team not. You can test it for yourself go and troll 10 match in a row and don't get ban, write "kys" or other shit and you will get permabanned :)
It's happen in game where you can acctualy "MUTE" all things from people, pings, chat, etc.
Ofc flaming it's bad, but you should ONLY get chat restricted for 10-50 games depending on what you wrote - but still a lot of people getting permaban.