Originally Posted by The Batlord

It's an American thing. Before the Civil War the states considered themselves mini countries who were all experiments in democracy with the right to their own particular vision (the United States "are" because it was a plural concept) and after that idea was destroyed by the war once it was made clear by the US government that this was a singular country where you couldn't opt out if you didn't like being a part of it anymore, meaning that states were not singular entities banding together for mutual benefit but a part of a whole (the United States "is").