its a legit problem and we know that riot does nothing against it.
Only people that don't know what Riot is doing say this...
Bots are a special case. It's EXTREMELY hard to get rid of botting in a game where making accounts is free. If you ban them as soon as they are spotted the bot makers will have an easier time making new bots that don't get banned. If you wait a couple of weeks bot makers wont know how they were detected.
And since botting is such a problem they want to make sure the most popular gamemodes aren't affected too badly.
So it's actually better to let bots infect aram, 3v3 and coop vs ai, because then they wont affect the majority.
In short: Botting is a problem that can't be as easily fixed as people like you think. They have to find a way of limiting the effectiveness of botting in a way that doesn't affect the players. It's complex problem.