The narrator in Leguin's lovely story is intrusive for the same reasons that the story's early sections has the overall construction it does: to keep the usual fictional dream from fully embracing us. Like some plays that break the third wall of the theater to shock the audience and keep them from treating the play as pure entertainment, these elements of the story should accept its constructed nature. As readers we should always be saying things like, "Why should this story be this way? Why should this society be this way?" In doing so, we act like the people in the story who challenge the nature of Omelas.
I think the narrator wants us to participate to build our own city of Omelas with our imagination. Omelas will be our own dream perfect place.You will notice however, after the narrator mentioned about the boy, she stop inviting us to the story. I think it's because that we suppose to be 'shocked' because then we will think what would we do if we were one of them. Would we stay? Or leave? In this way of writing, the author purpose to make the reader questioning about their ethical views and morality would be reached.