After the general justification that it is the child's misery that preserves the prosperity and happiness of the city, Le Guin lists several other reasons people give for keeping the child in prison. First, the child is already in such a degraded state as to be incapable of enjoying freedom. Then, the child's habits "are too uncouth for it to respond to humane treatment." Finally, the child might actually be afraid without the prison walls to protect it.
These justifications are all related, and Le Guin makes it clear from the context that they are merely excuses the people of Omelas make in order to feel better about their cruelty. They only begin to come up with these excuses after they have "faced this terrible paradox" and perhaps seen the child. Sometimes it takes them weeks or years to do so. Perhaps even more telling is the point that these objections apply only to the particular child whose life they claim has already been ruined beyond repair. No justification is offered for the practice of ruining lives in this way as a matter of custom. To accept this misery as the basis of their civilization can only be attributed to selfishness.