According to Sicko (Michael Moore's documentary) which I watched idly once, Moore's Canadian grandparents (I think they were his grandparents) always get travel insurance from their local department store before visiting the U.S., because of this very issue.
Apparently they did it every single time.

I'm not sure of how widespread this is (i.e., could be only old people, only Moore's family, mildly staged for the documentary, etc).