In a case such as this one, it might not have been obvious to the gate personnel that there was going to be an issue. Taking everyone else off the aircraft might be so they wouldn't film anything done to the dog, because that would probably cause more of a backlash than dragging the woman off.
One tactic when there's an animal in the cabin is to invite the other passengers on the row to board first so they're in place when the dog arrives. That makes it way harder for the dog owner to try to commandeer space because it's already occupied.
Any news on what breed of dog it was?