Case law is made by federal and state courts. Judges will interpret the laws or regulations and apply them to individual cases. Although the different states and the federal and state court systems are separate and do not control each other, judges often consider the opinions of other courts in interpreting similar laws or regulations. Federal regulations have been enacted to detail the responsibilities of businesses, employers, schools, transportation providers, and others under those federal laws. The most frequently cited regulation is that of the Department of Justice.
State laws are of two types.
Most states have enacted civil rights laws which may or may not mention assistance dogs. The other type of state law is criminal. The federal law is intended to grant liberal and extensive access to assistance dogs. The business can refuse to allow access to your dog if admission would fundamentally alter the nature of the service provided or would jeopardize the safety of the operation. The ADA is relatively new and we don't have case law or experience to tell us how this will work.