Why do I need power of attorney to discuss medical bills re my husband?

Doctors have become very careful about disclosing any sort of medical information to anybody, yes even wives. As Eyerish said you need a POA and HIPPA release to protect yourself and your husband. Is he competent? If not, you may need to pursue guardianship and conservatorship. Sad, but true

Because of federal privacy laws. Even between a husband and wife.

As his wife you should have POA anyway.