The reason that Reed's hit on Branch was penalized, and for which he will probably be fined, doesn't have to do with the part of the body he hit Branch with but the part of Branch that he hit.
You particularly can't hit what's defined as a defenseless receiver, which Branch was in that situation, in the head. Reed did. Penalty and fine. Plain and simple.
When you look at what happens to Branch's neck as a result of the hit, you can see why the NFL wants to eliminate this play -- he could have been paralyzed. The same hit to the chest or shoulder is completely legal.