Why the umpire should NOT signal TD

[BktBallRef]

Originally posted by MJT
When the umpire goes up with the TD call, the wings cannot say, “yes, but his knee was down before the ball crossed.”

Why can't they?

How is it any different from one wing signaling TD but the other comes in and says, "No, you couldn't see that his knee was down. That's why I didn't signal."? How is it any different?

What happens if the U "punches it in," one wing doesn't see the knee down, signals TD, and the other wing comes in and says "His knee was down.