Immediately after he has kicked the ball if a receiving team player runs into him it is either a 5 yard or 15 yard penalty. But at some point the receiving team has the ball and the punter might actually try to make a tackle and, I’m assuming, it is now legal to block him. When is that transition?
What if the punted ball isn’t fielded but allowed to roll and still hasn’t come to a stop? The punter well may be running downfield but the ball isn’t in the other team’s posession. What then? Surely he can’t be allowed to hit without being a target himself.
ETA: Hmmm… it says “unless contact is caused by the kicker’s own motions”. Maybe that covers it.
The NFL doesn’t publish its complete rule book, which makes it difficult to answer questions. The college rule is (9-4-a-5) “The kicker of a scrimmage kick loses protection as a kicker when he has had a reasonable time to regain his balance.” I believe the pro rule is similar, if not identical.
Despite the rule, there’s a fairly strong ethical standard that you don’t blast punters unless they’re really trying to make a tackle. Nobody wants to get their own punter clocked in retaliation.
I can’t access YouTube here at work, but there is a clip of a Philadelphia Eagles punter getting absolutely drilled. It was a legal block, but the commentators found the hit to be in very bad taste. If someone can please find the link and post it, great. If not, I will when I get home.