To add to what Jan said, the rockets themselves fire for only a very short time. The reason the Bazooka doesn't need a gunner shield is because the rocket has actually ceased firing before leaving the tube- thus the gunner does not need any protection from a rocket blast to the face. The Panzerschreck, on the other hand, traded the need for a shield with greater range and accuracy by extending the rocket burn time out until a very brief (like, few milliseconds) period after leaving the tube.