International Travel Insurance plans are normally for people traveling outside their home country. There are US citizens who are residing outside the USA and visit US for short durations visiting family, for holidays, business, during Christmas and Thanksgiving holidays or other similar occasions. US citizens who are in this category are not eligible for domestic healthcare in US as they are permanently residing abroad.
Some companies consider the US to be the home country for US citizens regardless of where they live and this makes them ineligible for the international travel insurance plans offered by these companies while visiting the US.
However, they do make exceptions for this specific situation with special conditions included. The common conditions include are that the traveller must be residing outside the US continuously for 6 or more months. Some companies might require the traveller to have a primary insurance already in force before applying for the travel insurance to visit US.
American Visitor Insurance provides international travel and health insurance plans for US citizens residing abroad making short term visits to US occasionally.