The veterinarian is doing the right thing according to New York's guidelines for veterinary practice, which you can find at http://www.op.nysed.gov/prof/vetmed/vetpg5.htm. Although the guidelines do not have the force of law, they represent best practices for veterinarians. See paragraphs 5.11 and 5.12.
Not only is the veterinarian protecting her/his own professional reputation, but they are protecting your cat from taking medication that isn't advisable or optimal under the circumstances.