I've seen people getting punished for sarcastic remarks and basic arguing about builds.
The ingame chat box is not the place for either of those things. The purpose of the chatbox is to strategize and help your team win.
If you're arguing in the chatbox, about anything, you're crusin for a bruisin.
Why are they not getting banned?
Because you're not told what punishments they're getting? For all you know, they might have gotten a chat restrict.
They are not flimsy. They don't exist. Riot bans "whatever".
Seems pretty consistent to me. It could stand to be a whole lot stricter, but again, seeing as how most players never get a punishment, and most of those never get another, I don't think the rest of the players are the problem here.