If you have an area where high-income receivers concentrate, you have a higher fiscal capacity. That fiscal capacity is a valuable resource and will create rent-seeking. People will trying to get that resource one way or the other, including immigration. It is very much like the medieval peasants putting their sheep on the commons pasture. It is better than the open range, and if you let them have open access they will, in fact, put too many sheep on the pasture and waste the value that the pasture has.