Answer:
All citizens because they have socialized medicine there. Many come here and pay because their wait for care is very unreasonable and American healthcare is unequalled.
The present American health care system is flawed. See the movie "sicko" by Michael Moore for a true understanding of the issues in the u.s. And why the Canadian healthcare system is far superior. It shows many Americans going to Canada for healthcare coverage because it is better and more affordable. Wait times in all hospitals and clinics in Canada are similar to the u.s. Including wait times for complex surgeries. Also you will never go bankrupt in Canada because all healthcare is covered including drug plans. Tommy Douglas, a Canadian politician, knew the realities of healthcare especially for senior citizens and he pushed for socialized medicine because he realized it is unethical to profit from sickness. Many Americans travel religiously to Canada to buy medicines that are reduced in price because the Canadian government bargains for medicine in bulk and obtains a bargain rate from the big pharmaceutical companies. In the us, the individual has no power to influence the high price they pay for healthcare or medicines, fees and prices are not kept in check. Hopefully for the u.s. That will change now that Obama has brought in reforms to the healthcare system.