It's not the "actual" fair catch. It's where the player stands and puts each arm horizontally to their side. (See the pose that he took towards the SCjr sideline as example; that's what he was doing to call them out).
So, it's not an official fair catch sign that means the umps would call the play dead. It's a non-official signal that the entire receiving team takes to signal the coverage team "There will be no return; you will not be blocked; don't hit anyone".
(That's how he explained it in his presser.)
Instead, all the SCjr players made the signal so that the coverage team pulled off. Then everyone blocked the unexpecting coverage team, and the return guy apparently returned the ball for extra yards.
(I didn't see it. Just going off of what he said in his presser.)
If it's true, I have no problem with him signalling the other sideline to call them out. It's dangerous for everyone, in that moment and in potentially every future kickoff SCjr returns, as the opposing team can't trust the unofficial signal and have to make the choice between being hit/blocked when they pull back, or knock the hell out of someone that is taking a legit fair catch.
If all that was true Shane is a dooj-canoe. He's teaching his players to be unsportsmanlike doojes. And he deserved even more call-out.
Also, based on Shane's curt explanation of his anger, and the outrageous reaction on-field... He knew he'd pulled a major dooj-card, and he was just waiting to go unhinged if he were called on it.