It is because we don't make these calls often enough that this crap happens.
As I stated, non-players are not permitted on the field at ANY time during the game - even when the ball is dead (between plays, time outs, intermissions, etc.)- without our permission. We have full authority to require all coaches/trainers/water carriers, etc., off the field for 100% of the game, if needed. Also, as I stated, we should 'work with' the teams to keep them under control at the sideline. If an OC or DC wants to take a step or two onto the field to give instructions between downs, fine. Just keep it to a step or two, give your signals, then get back behind the white.
In this particular case, the HC obviously believed he would get called for being on the field, and incorrectly believed he would only get a warning, under 9-2-5, which would bail his team out of a DOG foul. Letting the play clock expire and penalizing a DOG would have been an acceptable 'compromise' resolution to this situation. Going all the way to 9-1-2-b would have been justifiable, even if a little excessive. But, something more than just a warning was needed.
These guys think they can do whatever they want, and if we do nothing about it, they have no reason to believe otherwise. And then the crew next week has to put up with this crap all over again.