If it's born on US soil it magically becomes a US citizen. 'US citizen' has always been a mystical category anyway. Until the current wave of PC crap you has a pretty good idea what an Englishman, Frenchman, German, etc looked like and what sort of cultural background they had. A US citizen can be anything vaguely human.
The concept worked for a couple of centuries when you could assume they were some sort of northern European and at least paid lip service to a branch of Christianity, plus the resident blacks of course.