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The author speaks directly to the reader about this:

The trouble is that we have a bad habit, encouraged by pedants and sophisticates, of considering happiness as something rather stupid. Only pain is intellectual, only evil interesting. This is the treason of the artist: a refusal to admit the banality of evil and the terrible boredom of pain.

She laments her inability to make Omelas seem like a real place, to convince the reader that such a society could actually exist, and invites the reader to try in her stead:

But I wish I could describe it better. I wish I could convince you. Omelas sounds in my words like a city in a fairy tale, long ago and far away, once upon a time. Perhaps it would be best if you imagined it as your own fancy bids, assuming it will rise to the occasion, for certainly I cannot suit you all.

Finally, after some more description she again directly speaks to the reader to ask them if Omelas seems real:

Do you believe? Do you accept the festival, the city, the joy? No? Then let me describe one more thing.

When the answer is “no” they add the detail of the suffering child, which is necessary for all good things that occur there. How exactly torturing a child results in the city’s scholars being smart or ensuring good harvest is not explained at all, and yet by some narrative alchemy the setting is transmuted from something meaningless into something interesting.

The non-subtextual point of the story is that we as a people cannot imagine even a fictional setting without injustice. The subtextual point of the story is that we cannot imagine a society without injustice, fictional or not. Just as the people of Omelas described in the last section convince themselves that the injustice of their society is necessary, inevitable, and futile to fight against, so to do we convince ourselves that the injustices of our society are the same way. And yet there is some hope offered in the titular ones who walk away:

The place they go towards is a place even less imaginable to most of us than the city of happiness. I cannot describe it at all. It is possible that it does not exist. But they seem to know where they are going, the ones who walk away from Omelas.

The author admits that she herself cannot imagine “the kind of place they’re going to”, in other words the kind of society that is not based on exploitation but is not an impossible utopia like the Omelas described in the first section, something that could exist in the real world (it seems the author failed to convince even herself). Nevertheless these people who are not her “seem to know where they are going”. This is an invitation to the reader to try to do what she couldn’t by herself: figure out how to structure such a society.

So, you can see what I mean when I say that its funny that this story that laments our inability to engage with anything but suffering and exploitation, is engaged with almost exclusively by talking about the mechanics and moral implications of the suffering that it uses as an example of this very tendency.