You are right that 14-day is the last punishment before receiving a perma. So keep this in mind, anything you say or do that is punishable in any way will result in a permanent ban, even if it is something as light as sarcasm or passive aggressiveness.
With regards to whether it is fair or not, Riot tends to punish more harshly for things that they consider zero-tolerance.
Namely: Intentionally feeding, using slurs that are racist, sexist or homophobic, and using language that encourages other players to harm themselves.
It's severely harmful to other players to exhibit such behaviors, and Riot thinks you should already know better than to do them. It's common sense/decency to not act so terribly towards others, so they punish it more harshly right away.
If you were given a chat log with your punishment then you can post that and we can help you identify what might have triggered a 14-day ban instead of a chat restriction. If you weren't banned for chat offenses then it was something else like Account-sharing or rank-boosting, or 3rd-party programs.