The relevant portion of the rule is: "It is a foul if a player initiates unnecessary contact against a player who is in a defenseless posture.

(a) Players in a defenseless posture are: [. . .] (2) A receiver attempting to catch a pass; or who has completed a catch and has not had time to protect himself or has not clearly become a runner. If the receiver/runner is capable of avoiding or warding off the impending contact of an opponent, he is no longer a defenseless player; [. . .] (b) Prohibited contact against a p

layer who is in a defenseless posture is: (1) Forcibly hitting the defenseless player’s head or neck area with the helmet, facemask, forearm, or shoulder, regardless of whether the defensive player also uses his arms to tackle the defenseless player by encircling or grasping him; and (2) Lowering the head and making forcible contact with the top/crown or forehead/"hairline" parts of the helmet against any part of the defenseless player’s body." Pierre Thomas caught the ball, to get down to protect himself, but was drilled in the side of the head on a helmet to helmet hit. I don't really care about the Saints or the 49ers, but it seems like that was a no call that cost the Saints the game. Not only was it a no call, it was a ruled a fumble (which it was), that resulted in a 7-0 swing for the 49er's. That 7 points on that no call was the difference in the game. I guess that's the real home field advantage.