Legal Question in Employment Law in California

Why should it be illegal? The employer has a right to know why you are not at work and when you anticipate coming back to work.

Legal? It is certainly not a crime. It is certainly within their rights to verify your story, including requiring you to provide doctor notes, checking up on you if you claim to be home sick, etc. That is true whether it is in the company policy manual or not. Keep in mind you are an 'at will' employee that can be fired any time for any reason, so keep your employer happy with you.