United States is a plural term

“The central fact of American federalism is worth repeating: the United States is by and large an economic union, by and large a social union, but not a legal union, or at least not completely. State laws are, or can be, rather similar, but this is, first, because the states choose to harmonize their laws, and, second, because conditions in the states are fairly similar. A state is free to be different (if it wishes), within its zone. But since the 1860s, the central government has gotten stronger and stronger, and there has been a steady, marked change in relations between states and the federal government. It is obvious why this took place. Changes in technology and socioeconomic structure paved the way. In the age of e-mail, cyberspace, satellite communication, and jumbo jets, the country is a single entity to an extent undreamed of in 1787. When all is said and done, however, the states still maintain a substantial reservoir of power.” – Lawrence M. Friedman, American Law: An Introduction