They don't just turn in their badges, next time we see them, they directly go to the slums, and deputize any of the battle-capable squatters into helping them protect the area.
Sergeant: Folks, in about forty five minutes, the Rehabs are coming to blow you out of your neighborhood. I want you to take your kids, and everyone that shouldn't be fighting to the basement. The rest of you are hereby deputized by the Detroit Police Department. Its time to show how real cops kick ass. (cocks shotgun)