Why is the plane of the goal line not applied on punts? Ball can break the plane on a punt and player can knock it back into the field of play and it is considered not to be a touch back. This seems to be inconsistent with a touchdown breaking plane. For that matter, it is inconsistent with the rule for a safety where the ball must be completely past the plane of the goal line in the field of play not to be considered a safety.
In the rules of the NFL, a touchdown is scored when a player crosses the goal line with control of the ball. As the NFL has interpreted it, the play is over as soon as a player running with the ball "breaks the plane of the goal line". However, you'll note that passes into the endzone require the player to make the catch, the play is not over simply because the ball crossed the goal line.
Punts are similar. You'll note that punting team players need to knock the punt back out of the endzone without touching the endzone. If they touch the ball while in the endzone, it's considered to be down in the endzone and therefore a touchback to the receiving team. Hence, this is why they leap into the endzone to knock back a punt.