In the US a large number of incarceration facilities are ran by the county police (otherwise known as the Sheriff's department).
The majority of jails in the US are run by county sheriffs or local law enforcement. The majority of prisons are run by the states.
The mindset behind being a good cop and a good prison guard are totally different. Exposing hires to the corruption and violence inside prisons right before sending them to police the streets creates a very adversarial mindset against the public.