Don't know about you, but I'm a citizen, not a subject. So were the founders. They rejected English common law, except for some definitions. But probably not this one.
The difference is, a subject can never become the sovereign. A citizen is the sovereign. The point of the "Natural born citizen" clause is to protect the nation, that is the people. The point of the Natural Born Subject laws and court decisions was to provide more subjects for the King.
Are you being intentionally ignorant?
We are discussing here the meaning of an adjective. The OED (You should look at it sometime in a library, if you ever go.) is a history of the English language and so gives examples of when specific usage of words and phrases entered the language. In order to show how an adjective is used, you kinda have to use nouns. (Like, duh.) The relevant usage examples of natural-born and native-born entered the language as modifiers of the noun subject.
ML/NJ