I have seen other refs call a time out when they NOTICE a fighter does not have their mouthpiece in, did exactly what Steve did....timed out, washed off the mouthpiece, put back in and continue fighting.
I mean atleast be somewhat humaine about it, a shot to the mouth WITHOUT a mouthpiece could shatter teeth, destroy gums, etc....not saying this hasn't happened in the past but, when the ref sees their mouthpiece out, they are allowe to stop the fight and have the fighter put it back in.
The sport doesn't need anymore unwanted negative attention if a serious injury was caused by something like this, and the fighter was not allowed to put his protection back in.