They just do, tbh. It's the same in normal games.
A few times (during norms. I don't play ranked because of the toxicity), I wasn't able to connect because my wifi got screwed over. Then when I got into game, it was about only 30 seconds in, but i still got a warning.
I assume it's the same reasoning or similar. And riot (or whoever is in charge of connections and getting players to play) can't really tell if you're dc, afk, dodge, etc. So i guess you can't really expect them to not take away LP because they have a feeling you dc'd or couldn't connect, cause they don't know.
That's what I think anyway