Not while the dog is working. Petting can be distracting. Like people, some dogs have food allergies, and some must be fed a special diet prescribed by their veterinarian. Most people enjoy talking to members of the public about their Service Dog. That could endanger the working team.
Federal law protects the privacy of individuals with disabilities.
They are not required to explain their disability, nor are they required to explain or demonstrate why they are accompanied by a Service Dog.