They do not have to make a rule. They arent required to. The game wont allow you to do it so there is no need for a rule, because it is impossible. Unless you hacked it to allow you to join a fleet in less time than the penalty, in which case there is a rule written to disallow hacking. You decided to make a squad, thats your choice. There are consequences for that when you decide to leave it though, and you are facing those consequences now. It is not SDEs fault, it is yours.
I will raise the BS flag, its definitely not game mechanics, a player can drop their squadron tag and join a fleet right away and they dont have to be a hacker, its built into the game to do so. So the other point I brought up that it should be a written rule still stands otherwise players do not know because players such as the one quoted above dont know what they are talking about. I dropped my squad tag and joined a fleet in a couple minutes. Now I am enjoying the game more.