Remember, in those days, it would be a rare thing for someone to have their child in a different country. Traveling was hard, and expensive.
If you went back to England to have your child, it could well suggest you had close ties to that country.
You didn’t just run around the world vacationing. And you didn’t tend to get jobs in different countries as a matter of course either.
Nowadays, the ordinary citizen has a reasonable chance to spend time in many different countries. The world is now global.
But “Natural Born”, does not mean “location of birth”, it means the natural state of the newborn.
The fears of allegiance are mitigated if you have American parents, and are raised in America, regardless of what country you managed to get birthed in.
See my post here, especially when I talk about my nephew's wife and her birth in Germany while her father was stationed there.
http://www.freerepublic.com/focus/chat/3393127/posts?page=138#138