Yes, should be either "Policemen work for the police" or "for the police force." The word "in" makes no sense.
I said "on" and it was marked wrong, but that's often how it's expressed in English. "He's on the police force."
Is "שוטר" a direct equivalent of "constable" or can it have other applications?
I think the point of this sentence is to try to show the difference between the Hebrew word for the individual police person and the entity in or on or for whom that individual works. The English sentence makes complete sense to me, an English native.
I was completely unaware that policemen work in the police force! I thought they work in the fire department!