Just so I understand, this is what your comments amount to.
1) In order to avoid injuries to a player who is distracted from the oncoming hit because he is focussing on the pass coming his way, the league considers him defenseless, and you cannot hit him in the head or neck area.
2) A player who can be prepared for the hit is not defenseless.
3) The offensive player is responsible for knowing the hit is coming while he is trying to catch the ball, and therefore has to choose between the ball and bracing himself for a hit to the head or neck, because the defender has every right to hit him in the head and neck so the receiver should be smart enough to forget about the football and choose to protect himself, because we wrote this rule to protect the receiver, but decided that we really don't want to.