Legal Question in Constitutional Law in Texas

Your question is about the English language, not the law. We're free to call people from neighboring countries "North Americans" if we want to; the law neither requires nor forbids that description. It is simply not an issue the law is concerned with. Similarly, you're free to use the term "U.S. citizen" instead of "American" if you prefer.

Our legal system sometimes treats U.S. citizens differently from citizens of other countries, but it rarely if ever cares whether somebody is a citizen of a country in North America or in another continent. With very few exceptions, the law doesn't distinguish between continents just as it doesn't distinguish between hemispheres, altitudes, tectonic plates, etc.