And the rules specifically consider a punt. Why do you think they didn't do this with fumbles?
Fumbled balls are downed at the point where they go out of bounds.
Not true.
A fumble that goes forward and out of bounds will return to the fumbling team at the spot of the fumble unless the ball goes out of bounds in the opponent’s end zone. In this case, it is a touchback. http://www.nfl.com/fans/rules/fumbleML/NJ
Because it wasn't necessary, or because this is just a "digest" of the rules, not the complete canon. Let us cite the rules digest which talks of the punt:
Examples of Safety: (a) Blocked punt goes out of kicking team's end zone. Impetus was provided by punting team. The block only changes direction of ball, not impetus.
Note how it does not say that the punt blocker provides impetus to the ball, but that this is one given exception to the impetus rule. That is how you are treating it.
It says "the block only changes direction of the ball, not impetus."
Again, by analogy, a defender punching the ball out of a player's arm does not provide impetus. So the rest of your argument fails.
It all hinges on what "impetus" means, which is what I said in my first post.
Fumbled balls are downed at the point where they go out of bounds.
A fumble that goes forward and out of bounds will return to the fumbling team at the spot of the fumble unless the ball goes out of bounds in the opponent’s end zone. In this case, it is a touchback.
OK, my mistake. The point remains. The offense can not advance the ball by a fumble because we do not want to reward throwing the ball out of bounds. That makes sense. The offense can never earn yardage on a fumble (out of bounds).
You suggest that the offense shuld never be penalized yardage either. I fail to see why that should be the case.
A fumble is a tremendous screw-up. It should have consequences. Fumbling around your own end zone should have even higher consequences.
This is football, not a nanny state.