DJ,
It is not illegal to hit them just because they are vulnerable. If a player times it right and hits a
PR after a catch of a punt, then that is legal. We all know that leading with your head is illegal and other hits might be illegal, but how vulnerable they are is not the issue. If the
PR does not want to get hit, do not catch the ball or call for a fair catch and you will guarantee you will be given some protection.
Peace