I think of it as:
"The (police) officers work for the (police) force."
I notice this course uses a lot of sentences that sound senseless in English, like "I have four ducks and the ducks," but the Hebrew version has gender so it doesn't sound like a crazy person talking, it's just specific in a way English isn't.
I think "in" works fine for "serve", as in "serve in the military", but "work in" just doesn't sound right. It should be "work for" the police or "serve on" the police force. "Work in" would be OK for the place you work, such as "I work in the police station", although "work at" might be better.
In the military is referring to the organization as a whole. You can work for the military in a variety of roles. It's not equivalent. Police force refers only to police officers, not the organization. A military version would be Navy seals work in the Navy Seals. Army chaplains work in the chaplains. A group of police officers = a force. You can't work "in the" people.