That’s the standard legal usage, defined since the Constitution.
Anderson ’55 [William; 1955; Professor Emeritus of Political Science at the University of Minnesota; The Nation and the States, “The Original Framers: Their Words and Intentions,” Ch. 4]
United States. Both the Articles and the Constitution utilize this term to designate the organized nation or the group of states taken as a single entity. In fact "United States" became the standard term at the time of the Declaration of Independence to designate the national unit of government and has been used in that way ever since.
There is, however, a subtle difference between the Articles and the Constitution in the way in which "United States" is used. Nearly every use of the term in the Articles is in the phrase "the United States in Congress assembled," as if the United States did not exist except when the delegates from the several state legislatures were assembled in a Congress. Usually the words "United States in Congress assembled" are not even capitalized. Clearly the term "United States" did not as yet mean a fully united nation or a political entity with an established name, a government, and a being of its own.
On the other hand, the framers of the Constitution, just ten years after the Articles were drafted, spoke of the United States and the people of the United States as if they already existed as one nation. "The government of the United States" is recognized as a distinct government, in Article I, section 8, paragraph 18, of the Constitution, and evidently as something separate from and not dependent upon the state governments; while the Congress of the United States is spoken of as such, and never as "the United States in Congress assembled." In contrast with the term "the United States" the framers used the phrase "the several states" to refer to the separate states and their local or particular governments.