On the one side, the USA is just one country. Logic says it should be, then, singular, just like the United Kingdom is. Example:
The USA owns this domain.
On the other side, if I however expand "the USA" to "the United States of America", I'd tend towards using plural — the noun the verb agrees with, "States," is definitely plural. Example:
The United States of America own this domain. → The USA own this domain.
What form should I prefer?
Best Answer
Short answer: in contemporary English, both USA and the long form United States of America are treated as singular nouns.
Long answer: Language Log has documented this in great detail. In the 18th and much of the 19th centuries United States was treated as plural, but in the latter half of the 19th century the singular usage became more common. Today, the singular usage is the only accepted usage, except for the case of a few fixed phrases. In fact, "in 1902 article in the Washington Post reported that Foster's work (which evidently was reprinted as a pamphlet) had persuaded the House of Representative's Committee on Revision of the Laws to rule that the United States should be treated as singular, not plural."