This is more of a general answer to the question and a commentary on some of the responses.
I wonder how many historical empires used abbreviations for their names under the mistaken belief that since everyone there “knew” their empires would last forever, everyone would always know what they meant.
While the term USA may be understood by most of the English-speaking world today as the United States of America, perhaps less so in the non-English world, can anyone guarantee that in 100 years or so the situation will remain the same? Mind you, no one can guarantee that Wikitree will last that long, but the plan is for it to be perpetual.
As pointed out elsewhere in this thread, until 1961, USA could mean the Union of South Africa, especially to someone living in that country. Would it be wrong for them to use USA as an abbreviation for Union of South Africa in a Wikitree profile entry and if so, I challenge anyone to explain the difference between that error and the use of USA as an abbreviation for United States of America.
It is fine to point out that if the State is included in the name that would uniquely identify the place, but what if it is not? I see many cases where the country is known, but no lessor geopolitical entity is identified.
FamilySearch standardizes on United States for the USA, which, to me, smacks of arrogance (as well as not being specific), as if there were no other countries in the world with that phrase as part of their name. Of course, they also use the make-believe entity they like to call “British Colonial America” which kind of demonstrates a different level of genealogical complacency to Wikitree.
As I continue my 40-year plus adventure into Family History research, I have become more and more convinced that in order to prevent confusion we should be adopting the attitude that place names should be spelled out in full as accurately as possible to avoid any possibility of confusion now… or in the future.