I understand how it might seem unfair, and that they are being treated differently, but there are reasons why one player is banned and another is not and things happening behind the scenes that isn't player facing (or at least not so readily visible to you).
Punishment for chat offenses climbs up through 4 tiers, starting with a 10 game chat restriction and ending with a permanent ban. Riot does not just immediately issue a permaban when a player is being negative in a game; they work their way up the tiers through frequent and/or severe behavior. An offense like the one you show won't result in any punishment by itself, it would take that sort of behavior on a frequent basis, and even then it would be one punishment tier at a time (not straight to a perma).
This might not be relevant to you, but it's up to the players in that person's game to report them as well; if they don't care, they don't have to report it and there's no reason to punish it. You also don't know if that person might be restricted soon or have been so recently, though if this is not his normal behavior it's unlikely he'll be punished.
I just want to help you understand how the system works and why not everyone needs to be punished, so if you want to post your chat logs for your ban we can go over them with you. Judging from what you said about "thin ice" though, it sounds like you simply continued to display negative behavior shortly after a 14 day ban.
