It falls under HIPPA. It is illegal for other reasons but HIPPA is specifically written for health care professionals. Look at the link you provided. It tells you exactly who it applies to. All the examples you gave are related to health care professionals giving out medical information. Again, Walmart was wrong. You are protected under many laws like the ADA and service animal law etc. But HIPPA was written to make sure that heath care professionals and associated businesses don't give out your medical information to anyone without your authorization. If Walmart asked your doctor what your medical condition was and the doctor gave it to them, the doctor then violated HIPPA. But Walmart asking you directly does not violate HIPPA because Walmart doesn't have access to your medical information, nor are they giving it out to anyone. They would violate service animal law, and if they did discriminate against you would be violating some parts of the ADA as well, but this in no way is HIPPA.