The reality is that Riot cannot ban all the actual toxic players, they would lose a huge chunk of their playerbase. They say that 0.2% (or something like that) of players are toxic whereas at least 2 people per 3 games are incredibly toxic. And that's lowballing it imo..
I mean if they banned THAT many people (almost 7% of the population) that's that many less people they have as a number playing the game, and the statistics would show like the game is dying. Where the reality of the situation is that it would actually promote more people to play as their gameplay experience would not be ruined as often, and in the event that it is, the offenders ruin less people's games before being removed.
But they do ban people who talk back to the actual trolls, and not ban the trolls/toxic people themselves, which is really sad. There are a lot of people who get trolled game after game (due to the high density of them in the population especially in ranked) and yes it will get to them (especially if they are in promos) and it's a downward spiral from there.
Like honestly if I had a Rioter secretly spectate my games and punish offenders, they could EASILY get 3-4 people per day permabanned and many more 2 week bans. And that's just in my games, other people experience the same thing dealing with these people. Sadly though these same trolls just ruin game after game after game and the next day, the next week, the next month, you will just happen to get them again and guess what? they will ruin that game too. It's sad how nothing is done about it when THEY are the true problem.
