Americans like to see themselves as part of an exceptionally prosperous and generous country. However, a minority of people are excluded from this good life of material prosperity. This is often dismissed by using a utilitarian argument. Utilitarianism is a practical, not idealist, philosophy developed by men like Jeremy Bentham that calls for the "greatest good for the greatest number."
In Omelas, people know and accept that their good life is based on the suffering of another. They simply dismiss the suffering child as the way life is and an unfortunate but decent trade off for everyone else's happiness. We in the United States could be said to do the same. We know that our good life is based, at least in part, on the sufferings of people in other parts of the world, such as children in Africa who essentially perform slave labor in mines so that we can have our consumer goods for low prices. If one is white, they know (even if they try to deny it) that the system is weighted against people of color, which helps ensure that whites have better lives. We know there are many people in prison, perhaps for the slimmest of crimes, who do work for extremely low pay so that we can buy lower cost goods and are often subjected to abuse. We can dismiss this or we can decide to do something about it—or we can leave the country, as do some of the Omelas people.
Le Guin argues that is wrong to base the happiness of the many on the suffering of the few, even if it is only one innocent who suffers. She argues too that nobody can really be happy under this circumstance, because the "happy" are always trying to rationalize or deny the unhappiness of the suffering one or few. American identity as a culture of prosperity and equality and as a society that aids the spread of global "freedom," Le Guin is saying, is stained by this contradiction in tolerating the suffering of a few.