Probably nobody thought it would be a potential problem. A close examination of definitions may help shine some light. A defenseless receiver is one that is attempting to catch a pass. If the pass hits the ground it's a dead ball, thus the player is no longer attempting to catch it and is no longer defenseless. Strictly from a rules perspective, any illegal contact after that point would be a late hit after the play. This is where over-announcing can get a Referee in trouble. If you just announce "After the play, Personal Foul, Unnecessary Roughness, #XX, Defense. The offense failed to make the line-to-gain and team XX will keep the ball. First down, team XX.". Then you leave no room for argument.