Obviously, "no abbreviations" is an arbitrary thing you just made up and is not the rule. It would make "Who is MLK?" and "Who is Martin Luther King Jr.?" both wrong, along with "What is the U.S.?" and "What is the USA?" A person wanting to write down "What is UPS?" would would be frustratedly ambivalent between putting down that or "What is United Parcel Service?" (Let's not even get started on "FedEx".) It's a path so pointlessly narrow and so patently ridiculous it's not worth considering.
If the clue asks for a country, there is no ambiguity between Canada and California and calcium. I can see that a response might be deemed unacceptable if an abbreviation was commonly used for two things of the same type, but that is rare, and not the case here. I mean,
"US" stands for a plethora of things besides the country in various contexts, but I think we can all agree that the show has never, will never, and should never turn down a response that uses "US" for "United States". If you google "What is the abbreviation for Canada?", you get a resolute Ca. If you google "What is the abbreviation for Thailand?", the answer is that the two-letter country abbreviation for Thailand is TH, and the three-letter code is THA.
I don't understand under what consistency of rules you can accept "US" for United States but not "TH" for Thailand. I think the
ad hoc rule you actually have in your head is "I don't see that very often myself, or have never seen it, and therefore I would not accept it." The rule isn't based on anything objective.