There is no certification or registry. In order to have a service dog, however, you need to have a documented disability and the dog needs to perform tasks which help mitigate that disability. In some situations, such as flying in a plane, or living in no-pets housing, a doctor's note is required.
The Department of Justice handles cases of fraud and it is illegal to claim your dog is a service dog, if it isn't.
There ARE laws regarding service dogs, just no national registry.
Service Dogs are legally defined as "dogs that are individually trained to do work or perform tasks for people with disabilities." It also says right in the ADA that "Service animals are working animals, not pets." How do you expect people to take your working dog seriously if you feed your dog table scraps and let him sit next to you at a restaurant? And how on earth do you not see how unsanitary it is for a dog to be seated at the table?
The dog also may not alter the environment that it is in. By sitting in a train seat or airlplane seat, the dog is taking that seat from someone else, and, because you are not allowed to charge people with disabilities extra because they requre a SD, that means lost revenue for the business. That is altering their environment.
For more information about SDs:
http://www.ada.gov/service_animals_2010.htm
http://servicedogcentral.org/content/
Just a note: Service Dogs do not have rights, the disabled handlers who require service dogs to help them lead a normal life do. The ADA was created to protect people with disabilities. Anyone who says something like "My dog has a right to be here," is either a faker or someone who doesn't know the law well. When someone handles their dog poorly, it threatens the rights of all of us who require service dogs. "Shorty" handles his dog poorly in public.
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