You are right in that there is no enforcement other than multiple billing attempts and an appeal to the patient and family to honour their obligations. But sometimes there is a bigger price to pay. For example, my ex-BIL had dual citizenship, US/Canada, but lived most of his life in the US. While in the process of relocating to BC he had a heart attack requiring open heart surgery complicated by kidney failure. The BC hospital provided the surgery and dialysis even though he had no medical coverage or finances. They then ambulanced him to a hospital in Blaine, WA as he was essentially deemed a US citizen. The WA hospital stopped the dialysis once they confirmed he had no way to continue the treatment. He died a week later. It was a very sad situation for my nieces but sure highlighted the differences in health care systems. I hope that most travellers to our country would ensure they had medical insurance.But there is no enforcement mechanism if someone refuses to pay due to not having insurance.
Airlines are providing free Covid insurance now when you purchase a plane ticket
Air Canada is providing "up to $200,000 CDN" (or $152,000 USD) for COVID insurance if you book an international flight (including the US) prior to the end of this month for a maximum 21 day stay. I don't feel confident that would cover the costs of a hospital stay in the US if I developed a severe case of COVID.