Why can China keep its currency, the yuan, artificially low?

If the Chinese yuan is valued low, then that's a good for American and European consumers. It means that products they buy from China are less expensive, so they can buy more. Walmart, Tesco, and Carrefour are full of reasonably priced products from China. If the value of the yuan goes up, then prices for just about everything will go up in America and Europe. If the yuan goes up 50%, then the price of everything at Walmart will go up 50%. Do you really want that to happen?