Why is it the duty and the privilege of the accused person to choose a door in "The Lady or the Tiger"?

In the rather whimsical short story, "The Lady, or the Tiger?" it is both a "duty" and a "privilege" for the accused to select one of the two doors in the arena because as one charged with a crime, he is constrained to select a door (duty), but the accused also has the opportunity to live and be married (privilege) and escape execution.
It is an odd form of justice that the semi-barbaric king of Stockton's narrative employs. When a subject of this king is charged with a crime, public notice is given and the "loyal subjects" assemble at the huge amphitheater that is the agent of the king's "poetic justice." With everyone seated, the king signals and a door beneath him opens. Out comes the accused subject into the arena. Opposite of him, there are two doors. Behind one is a hungry, fierce tiger; behind the other, a lady. If the door to the tiger is opened, the guilty subject is savagely torn to pieces by the tiger, and public mourners weep and cry out for the loss of the subject. However, if the door to the fair maiden is opened, the subject is immediately married. ("It mattered not that he might already possess a wife and family.") Celebrations begin, bells ring, and the now "innocent man" leads his bride to his home. With irony, Stockton writes,

This was the king's semi-barbaric method of administering justice. Its perfect fairness is obvious.

At any rate, the uncertainty of the situation certainly entertains the people.