I'm assuming it's the mayor's way of getting back at the cops for their refusal to police the city after she arrested some of them for it. They will bring in feds to do the work and to report on those who are looking the other way at local thugs. She'd rather federalize the city's police than make amends with the cops.
But the cops will just show up to work every day, collect their pay, stay out of dangerous situations, and arrest a benign perp once in a while to make it look like they are doing something. If that is so, then their lot has downshifted considerably in life or lawsuit danger for the same money. I don’t think they’ll mind that one bit unless of course their jobs are threatened.