Actually they may be. If one is using Vattel's rule in "law of nations" for what a natural born citizen is, one should also use the exception for children born in the armies of the state, which also includes the diplomatic service.
Book II
§ 212. Citizens and natives.
The citizens are the members of the civil society; bound to this society by certain duties, and subject to its authority, they equally participate in its advantages. The natives, or natural-born citizens, are those born in the country, of parents who are citizens.
§ 217. Children born in the armies of the state.
For the same reasons also, children born out of the country, in the armies of the state, or in the house of its minister at a foreign court, are reputed born in the country; for a citizen who is absent with his family, on the service of the state, but still dependent on it, and subject to its jurisdiction, cannot be considered as having quitted its territory.
So while a military base in a foreign land is not US territory, and a child born of a non-citizen on such a base is not a citizen of the US, children born to parents "in the service of the state" are natural born.
§ 217. Children born in the armies of the state.
For the same reasons also, children born out of the country, in the armies of the state, or in the house of its minister at a foreign court, are reputed born in the country; for a citizen who is absent with his family, on the service of the state, but still dependent on it, and subject to its jurisdiction, cannot be considered as having quitted its territory.
http://www.constitution.org/vattel/vattel_01.htm