The rationale has been that, since one can't predict who will be sexually promiscuous or simply unfortunate in choice of partner and who will become an injecting drug user, it's best to vaccinate when the opportunity is there. Also, because of the possibility of lack of maternal obstetric care, hence perhaps no documentation of maternal Hep B status, the newborn could be at risk for Hep B as an infant, a serious disease. Big bureaucracies write policies broadly, and so we got universal infant Hep B immunization, at birth.