They didn't always. The court in Minor v Happersett recognized that other authorities disagreed.
What do you suppose the effect was of a well known legal book constantly misinforming people on this particular point since 1825? Add to that the additional confusion of how many people simply thought we adopted British common law regarding "subjects" and applied it to our "citizens"?
The earlier cases seem to know the difference, while the later cases went increasingly over to William Rawle's view.
Thanks for the link. Unfortunately, Long was not a member of the Federal Judiciary.
But Charles Evans Hughes certainly was. I seem to recall John Marshall also believed Vattel applied to citizenship.
Re: 181 - well, someone needs to put together a compelling argument and who also has standing.