Why is a goalkeeper stepping off his line on a penalty kick and saving the ball not considered DOGSO-F since his actions clearly denied an obvious goal scoring opportunity by committing an infraction that would have resulted in a free or penalty kick?

USSF answer (July 13, 2011)
All infringements of Law 14 are punished according to Law 14 itself. When any member of the defending team violates Law 14 (of which the goalkeeper moving illegally is one example), there are only two possible restarts — a kick-off or a retake of the penalty kick.

If the restart is a kick-off, it means that the interference was not successful and therefore a red card for denial of an obvious goalscoring opportunity is not permitted. If the restart is a penalty kick, it is a retake of the original penalty kick, not a new restart, and thus it also does not come under the requirements for dismissal for denying an obvious goal-scoring opportunity to an opponent moving towards the player’s goal by an offense punishable by a free kick or a penalty kick.