The US Military hires contractors to do basic security work in a wide variety of areas. Could they do it themselves? Yep.
Could they do it for the same price? No.
The four security guards are probably part-time with no benefits or other good stuff. They would probably have to spend 3x as much to do the same job with real cops. If they really have a need for security guards at the police station (unlikely), the rent-a-bobbys are a cheap solution to the problem.