Why is place of birth a determing factor in hiring and placement? Is there a magical fairy that comes down and taps you on the head with the loyalty stick if you are born in the US?
There is more of a chance that a naturalized citizen will have loyalties elsewhere. They have a native country and an adopted country whereas an American born has just the native country and would need to adopt a country to spy for.
There is also a better chance that the immigrant brings with him from his native land ideas, beliefs or customs that are foreign to this land and those previously assimilated beliefs might conflict with his being loyal to his adopted country and instead identify with enemies of his adopted country.
It's amazing to me that someone would believe that the native and the naturalized citizen would have the same propensity toward disloyalty. If that is what you are asserting, please make your case.