I agree with the previous two posters. The use of a third person narrator ensures that we as readers don't identify with the specific thoughts and emotions of any one character. Imagine how the story would have been affected if we as readers new what was going through the minds of the children as they stoned the grownup. It would not have had the same impact.
To add to the above, the objectivity of the narrative perspective allows the reader to view the events in the story as they stand. There isn't a moral to the story, per se; however, morality is questioned by the events and outcome of the story. Jackson calls into question the nature of tradition and our adherence to tradition which is a moral issue.
The narrator of the story is a third person impartial one. The narrator does not see into the minds of the characters and relates the series of events in a matter-of-fact way and does not comment on the events in the story. This type of narrator works well for this story because the focus is on the events that occur.
A moral is a lesson that is brought forth in a story. You must use this term carefully because not all stories have morals; theme might be a better term to use here. The main theme of the story is the inhumanity of violence and the apathy that people have concerning this inhumanity. Jackson wanted to bring attention to the fact that society had become so numb to violence and crimes against humanity that things that were clearly wrong were accepted.