Because ECG is a medical device, Apple does not want to have it product banned because it allowed ECG use in an unapproved manor.
Unless Apple got some certified proof that you were a citizen of a country that approved ECG, then it has to guess. It knows where it shipped the Apple Watch for sale and whether that country has approved the app, with the assumption that it is the citizens of that country that purchased the Apple Watch. It knows based on IP addresses where the Apple Watch is being used and if that country has approved ECG.
What it does not know is if the person using the Apple Watch is a citizen from a country that has approved ECG. So it is doing the best it can to not cause governments to ban the sale of Apple Watches because they allowed their citizens to use an unapproved medical product.