We can't do that without a fuller understanding what the NFL means by "impetus." It's not the same thing as physics class where a change in momentum means the object changes speed and/or direction.
A blocked punt changes speed and direction, but the NFL does not consider the punt blocker to have provided "impetus" to the ball.
By analogy, a defender who punches the ball out of a player's arm does not provide "impetus" to it either. That would be my understanding of the NFL rule and this is how the rule was applied on Saturday. I trust the NFL to know its rules.
Surely if Dallas thought the rule was applied wrongly there would be someone other than you asking this question.
I'm not sure it's "fair" either to award a safety to the defending team.
Fumbled balls are downed at the point where they go out of bounds. That's the rule. It is applied the same way in the end zone. But being "down" in the end zone means a safety.
It's logically consistent, and I see nothing unfair in having consequences to screwing up around your own goal line. What you suggest is either two sets of rules: where a ball fumbled from the 10 yard line back to the 1 is spotted at the one, while a ball fumbled from the 9 yard line back to the goal line is spotted at the 9.
Or a general change whereby any backwards fumble is spotted at the point from which it was fumbled. That is, fumbles carry no penalty as long as the fumbling team recovers.
Either way, it seems an attempt to minimize the offensive team's risks and insulate them from their own poor ball-handling.
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