I guess it is a matter of interpretation. Tomlin is one of the most meticulous coaches around. He is a perfectionist and is on the rules committee and has to know the rules. Just given the circumstances and his actions and reaction after the player went by maybe he had a momentary brain lapse or acted out subconscious frustration or impulses by crowding the field but he surely knew what he was doing.
Did Woody Hayes know the rules of society and the football field when he couldn't control himself and took a swing at the Clemson player? Of course. Would he like that one back? Probably. I have no doubt Tomlin didn't accidentally step on the field in front of the returner. No way. I coached youth football for ten years and even at that level your position on the sideline is hammered into your brain by the refs so no way an NFL coach is unaware of his surroundings on the sideline.
Simple solution to this. either start fining everyone who is in the white area whether they are involved in the play or not or start penalizing the team every time someone is in the white area. It's in the rules and they all know it is so hammer them on it. It's one of those rules that is not enforced.