The spirit of the halacha is that a women cannot be in a dominant position over men. The role of a woman is supposed to be more supportive and subservient. If not subservient, she must at least not be in a public position of dominance. She must work on the side lines, in private. This is tznius.
A women also should not be a police officer. It is not proper for a woman to arrest or detain a man. That is why it is assur for a woman to carry a weapon or dress like a man.