Do you really find it harder to believe a tourist wouldn't have travel health insurance? I've never heard of anyone buying that for a day trip to Canada. And just generally it's rare in my experience for people to buy insurance unless it's required in some way. Maybe that is an American phenomenon though.
By contrast, I know few Canadians who would set foot in the USA for one day without travel health insurance. We've all heard the story about the Canadian couple who went to Hawaii on their honeymoon and delivered a baby (prematurely) there, and it cost them 1 million in hospital fees because their coverage wasn't sufficient. And they did have insurance, it just didn't cover baby deliveries.
It sounds like it was the Canadian insurance provider who screwed them, though. They said[2] that since she had a bladder infection prior to getting the travel insurance, it was a pre-existing condition and therefore wasn't covered. So basically, if you get your travel insurance and come across the border, don't expect it to cover anything like a heart attack, an abcessed tooth, a prescription refill, etc.
