Why should the Chinese revalue their currency?

Tim Geithner has now taken up Henry Paulsen's weary lament to the Chinese that they should revalue the Yuan, last week accusing them of currency manipulation. Why is the US so interested in this? It will only mean that prices in Wallmart go up when everyone wants and needs low prices. What's the real reason?? The US did this with Japan 10 years ago and they conceded but never recovered. Why can't US leave China's economy to the Chinese - they can manage it quite well themselves and a lot better than US manage theirs??