Punt Returners

I don't understand why punt returners are allowed to block. They block to keep the opposing team from downing the ball, usually close to the end zone, so their team can start from the 20 instead of deep in their own zone. I don't think the punt returner should be allowed to block until after the ball hits the ground. Why? Because the players on the punting team are not allowed to hit the punt returner until he touches the ball. So, if the ball is in the air, the gunner can't knock over the punt returner to get him out of the way so he can down the ball. It's only logical ... which I guess explains why it's not a rule.