Local police forces in major cities are larger & more well armed than most militaries. A force like the NYPD, which is iirc the fourth largest military on Earth, if it actually ratcheted up its opposition to the feds, could escalate things into armed conflict *fast*.

Also the whole thing of forcing a sympathetic federal authority to crack down on state authority.

Yet another area where Black Lagoon took a basic trope & made the definitive edition of it.