The quarterback isn't the only person protected. Any player in a "virtually defenseless posture" is protected. Here's the rule --
( g) using any part of a player’s helmet (including the top/crown and forehead/“hairline” parts) or facemask to butt, spear, or ram an opponent violently or unnecessarily; although such violent or unnecessary use of the helmet and facemask is impermissible against any opponent, game officials will give special attention in administering this rule to protecting those players who are in virtually defenseless postures (e.g., a player in the act of or just after throwing a pass, a receiver catching or attempting to catch a pass, a runner already in the grasp of a tackler, a kickoff or punt returner attempting to field a kick in the air, or a player on the ground at the end of a play). All players in virtually defenseless postures are protected by the same prohibitions against use of the helmet and facemask that are described in the roughing-the-passer rules (see Article 11, subsection 3 below of this Rule 12, Section 2);
I haven't looked at the play again, but if Newman had just caught the punt and was hit helmet-to-helmet, it should have been a penalty -- UNLESS the refs said the Colts player was blocked into Newman.