If the FBI's main objective was law enforcement, then wouldn't the general populace expect them to pursue other Federal agencies that are actively breaking the law? Where as, if the FBI states their main objective as 'National Security' they can turn a blind eye to the the other law breaking Federal agencies as long as these agencies are acting within the interests of 'National Security'.

And in truth, many citizens are law breakers. And many of the laws that are broken today may not even exist in a couple of years. Where as, ensuring 'National Security' is a non-transitory objective.