How to adequately explain to those who don't know LoL that you can't leave the game once it starts?

technically your argument isn't exactly true, because in no way are we required to stay in game, just riot says if you DO leave, they will punish you.

having afk for the randomest reasons sucks, but people who are new to the game just simply don't take it so seriously yet, not until they get into the competitive scene that is. basically there isn't exactly a real way to explain to them the grief they are causing until they experience it themselves.