I could be wrong, but I think the NG is onsite as props. They lend the notion of authority and muscle.

I could be even wronger, but I doubt that they have real authority. They are not sworn police officers, for example, and I question whether they can even themselves make an arrest.

Since it doesn't seem they are there to keep the peace (I don't remember hearing of riots or civil disobedience at airport security checkpoints), and they are certainly not there in a military capacity (the posse comitatus act of the last century forbids the army from domestic civil police duties), they seem to have an unclear and only semi-defined sort of duty-- not really screeners, and not really policemen.