Why is AFKing a ban-able offense...

...when no one gets banned because of it?

From my understanding of the game, the reportable offense is (in exact terms) "Leaving the Game/AFK." This, to me, means that players leave the game, intentionally or unintentionally putting their team in an unfair 4v5.

I've had plenty of people AFK in my games. I've even had one come back in the post game lobby after the game was done! I usually report the AFKers who had a direct impact on the game (say, my team loses, but it was close and we would've won if someone didn't disconnect), but I've never had a notice that action has been taken.

Can someone explain if I'm missing something? What is considered a ban-able AFK? Are no actions being visibly taken because leavers are already punished in their client?

(PS. On a similar note, Riot might want to look at Ascension and other game modes which have high automatic gold gain and level gain. I played a game of Ascension where the enemy Kled was apparently AFK for the first half, which is true because I didn't see him at ALL on the map. He comes in halfway through with just as many items as everyone else...? It just feels wrong to have AFKers jump back into game modes with no penalty in-game.)