> I can understand how repeated interaction with a recalcitrant criminal element could harden and corrupt a police officer.

If a job is hard and hardening, and the job requires you to behave fairly, sensibly, and rationally, then a reasonable employer will often and regularly check to ensure that the job hasn't damaged the employee in such a way as to render them useless or ineffective.

One common way to handle this sort of thing is to proactively rotate employees in and out of the stressful portions of the job.