Makes sense if you consider nurses tend to be the ones administering the shots. Though I'd agree that anybody working among sick people should not be allowed to do so if they refuse shots. At that point it just becomes neglegent and perhaps dangerous.
From what I understand most nurses refuse to take them voluntarily.
The reason: Medical Licensing. The licence for a doctor is much more valuable than that of a nurse. So a hospital can replace a nurse much easier than a doctor. Also the labor agreements between nurses and doctors tend to be different where the nurses work directly for the hospital while doctors tend to be under a contract.