I think they focused on air travel because denying boarding to a COVID patient by the pilot is defensible as an extension to any pilots right to refuse boarding to any passenger who may pose any risk to the flight, with the COVID being defined by the government as a risk. Because this is done by the airline outside the country, it gets around the US citizens right of return to the US at the country’s borders, hence to lack of such testing requirement for land and sea entry. It’s a bit of a constitutional run around.