All country names that are plural words (die Vereinigten Staaten, die Niederlande, die Vereinigten Emirate, ...) also use plural verbs. It doesn't matter that you think of the country as a singular thing, it's just grammar. As soon as you switch to a singular word, you use singular again ("Amerika/Nordamerika ist sehr groß"). With the abbreviation, you can choose: you can mentally expand the abbreviation to the plural word, and then use plural, or use singular because factually it's a single country. A quick google search for "die USA ist" and "die USA sind" shows that both are about equally common. Personally, I use always plural verbs for USA, because I think "Staaten".

'The United States' is also plural in English. I'm only guessing, but it seems it stays plural in German when abbreviated.

Die Vereinten Staaten sind sehr gross. It's plural! Der Staat, die Staaten