According to the ENGLISH Version of Natural law, Because Ted Cruz is born in Canada, he owes his allegiance to Canada. According to the VATTEL version of Natural law, Because Ted Cruz's father was Cuban at the time of his birth, he owes his allegiance to Cuba.
So where would "natural law" support the argument that he is a "natural born" American? The Naturalization law under which he acquires citizenship was an artifact of 1934, not 1787. It was a gift from Congress, not a manifestation of "natural law."
You’re referring to common law.
Natural law is the law that is written on the human heart or, IOW, the Golden Rule or the Ten Commandments.
It’s application here is very remote, since reason does not tell us immediately which nation should claim Ted as a citizen.
Given that his mother was a US citizen, and his father was a Cuban citizen, it seems that either nation could claim him. But since the family had established residency in the United States at the time, it seems to me that the United States would have the greater claim.