The comparison isn't what happens to someone's job when they're facing a complaint relating to that job, but what happens to their job when they're facing criminal charges.
Garrity basically sounds like institutional corruption whereby the (administrative face of) the police to avoid asking questions that could help the (criminal-prosecuting face of) the police. At any other job, the internal investigation would demand answers to questions that would criminally implicate you (under pain of firing), and the police wouldn't agree to close their investigation due to the administrative inquiry beginning!
Also, these separate serial investigations seem like a powerful incentive to brush over the criminal inquiry as quick as possible - figuring wrongdoing can still be punished by the second investigation, and also not wanting to delay it.