Why can attacking players not be offside at a penalty kick?

Answer 1

A player cannot be called offside if he receives the ball directly from a throw-in. The reason for this is that the offside law was created to prevent a player from posting up near the opponent's goal while the play was taking place at the other end, because such behavior is considered unsporting and inconsistent with the spirit of the game.
The laws were written so as to promote the spirit of the game and sporting behavior, not to compensate for the tactical and strategic failures of the other team. All of the "out of bounds" restarts (throw-in, corner kick, goal kick, and kick-off (yes, goals are out-of-bounds events that also score)) do not have offside applied to players receiving the ball "directly" from the restart.

Answer 2

Proper placement of players for the kick dictates that a player may not be ahead of the ball. If they are, the referee will not blow the whistle to start play.