While some people might dress as a cop for Halloween, doing so for any reason besides trick-or-treating or going to a party constitutes a very serious crime.
That badge and uniform can grant officers access to resources and property they otherwise wouldn't be allowed anywhere near. With this in mind, it's no surprise that some enterprising thieves would bluff their way to these advantages and clear out before anyone's the wiser.
However, one particularly disturbing case from Australia highlights the danger that impersonating a police officer can pose to public safety.