I believe that the cause of the segregation is money and a sense of family. Many ethnic families come to America and move to a large city to find work. They find that not only are the nice parts of most large US cities incredibly expensive, but there is already an ethnic community in a certain part of the city that can provide comfort and support. It certainly isn't the US telling them, 'you're chinese, you have to live in china town'. The city I'm from has the third largest Arab American population in the US, and most of the Arab families live in the east end of the city, but the longer the family has been here (generation-wise) the more likely they are to move to the west end. In other words once the family is comfortable with their new city, they can leave their cultural community.