Well, compared to what?
The odds of having your private health insurance rescinded in the US is also very small -- but it's quite likely among people who suddenly rack up big health care costs.
Similarly, if you compare Canada-to-US medical tourists to the Canadian population, I'm sure it's amazingly tiny. But how does it compare to the people of means who have a medical procedure denied by the Canadian system?
(I don't fault Canada for denying care in some cases. Sometimes it's not worth it. But the Canadian system would be better if people were still allowed to top off the denied care with their own dollars and didn't need to leave the country to get it.)