I signed up for this site just to address this. Here's my take:
The correct call on the field should have been an illegal bat by Polamalu, with the Patriots given 1st and 10 with 8 seconds left ten yards in advance of the previous spot. Even after the officials failed to see the illegal bat, on review it should have been ruled a touchback, with the Patriots given the ball at the 20 with 8 seconds left. Rule 3-15-3(a) says: "Impetus is the action of a player that gives momentum to the ball and sends it in touch. The impetus is attributed to the offense, except when the ball is sent in touch through a new momentum when the defense muffs a ball which is at rest, or nearly at rest, or illegally bats a kick or fumble." Rule 3-28 says: "A safety is the situation in which the ball is dead on or behind a team's own goal line provided the impetus came from a player of that team and it is not a touchdown."
Presumably Carey did not overturn the play because illegal batting (12-1-8) is not a penalty that can be found on instant replay (very few penalties can be). But he didn't need to find a penalty, just rule that the ball was illegally batted. Suppose a ref wasn't able to rule this way; then the illegal batting part of rule 3-15-3 would have no purpose. If illegal batting was ruled on the field, then the result of the play doesn't matter because the penalty would be enforced instead. Under the presumption that all rules have a purpose, the effect of the rule is to create the possibility that a ball can be ruled illegally batted into an end zone *even if* the penalty for illegal batting was not called and cannot be enforced on review.
Under the new rule that all scoring plays are automatically reviewed, the elements of what constitutes a safety are reviewable, just like with a touchdown or field goal (ball in player possession, ball breaking the plane, ball legally kicked, ball passing over the crossbar, etc). The reason it would be a touchback for the Patriots is that the rules say it's not a safety, and it's also not a touchdown, and another rule says that any ball that becomes dead in touch is either a touchdown, a safety, or a touchback.
The other possibility for the intent of this rule is that it creates an option for the offense when something like this happens. Suppose the ball is snapped, the QB fumbles, a defender illegally bats the ball out of the back of the endzone, and a penalty is called. The alternate reading of the rule is that *if the penalty is called*, then the offense can accept it and repeat the down ten yards in advance of the previous spot, or decline it and take a touchback, getting a first down at their own 20. Thus you'd decline the illegal batting penalty if you snapped the ball from inside your own 10, and you *might* decline the penalty if you snapped from a few yards beyond the 10, the penalty would not give you a first down, and you value a first down more than the yards.
But I'm sticking with my first interpretation. Impetus is an element of a safety, and thus it is a reviewable play.