Having a mute button in game does not mean you can trash-talk as much as you like
This goes to all banned/suspended toxic people who thinks the mute button is their "free toxicity" back-up whenever they want to talk shit for no reason. If you think mute button is something that can allow you to talk crap in-game, then the WHOLE Lol player base around the world would be filled with nothing but r%%%%%s full of negative thinking in their head(and tons of players "on-tilt").
Mute button works as a gaming atmosphere "calmer" when a certain someone wants to have a good pleasant game with no negativity in the air, whilst also expecting excellent teamwork. There are also people who don't always mute toxic people for a good reason, which is hoping that the toxic dude would at some point ping out the danger/ward or anything related to relay to the team. Having the dude muted would negate the one important call & become unnecessarily decimated themselves.
As you can see, imo this is why even muted toxic people will still be banned/suspended even if they're trash-talking to themselves in-game. If they are not at any least helpful in communicating with the team well, then they are better off uninstalling League as there is no place for filthy scums like them.