The best reason to keep it quiet was posted earlier. They don't want thousands of people coming into the area looking for family members. That will hamper both rescue and recovery operations. It also increases the number people that will have to be cared for (food, shelter etc.).
But this is America. And I don't buy the argument that people who are looking for loved ones will less likely to come and look because they think there is a higher chance they are dead. And I am tired of having things kept from the American people. We need to know what we are dealing with so that we feel inclined to help.