You are speaking the absolute truth and talking from experience, this has been an issue since the beginning of LoL. If Riot hasn't fixed it yet it's because they don't want to. The fix is so easy that it would take half an hour of coding to implement.
Now, if you wonder why they don't want to fix it, that's a much harder question. The best explanation I can come up with is because the number of people going afk is so high that they would hurt their sales too much. When something goes to hurt their income they won't do anything about it despite what they write or what they make the community believe.
Justice and common sense dictates banning people who go afk repeatedly but these are probably hundreds of thousands, if not millions of players. Of these millions, some of them buy Riot Points and contribute to Keep Riot in business. So much that Riot doesn't want to piss them off and make them stop buying Riot Points.
On the other hand, there are players like me that refuse to buy RPs until Riot shows some kind of real action against toxic behavior and not just smoke and long posts in the forums to make people believe they are "working hard" to fix the issues.