When the Americans with Disabilities Act was passed and signed into law in 1990 the US Department of Justice was charged with writing the regulatory law to implement Titles II and III (state and local government services and public access respectively). The original definition of "service animal" in these regulations was:
"Service animal means any guide dog, signal dog, or other animal individually trained to do work or perform tasks for the benefit of an individual with a disability, including, but not limited to, guiding individuals with impaired vision, alerting individuals with impaired hearing to intruders or sounds, providing minimal protection or rescue work, pulling awheelchair, or fetching dropped items."
The phrase "providing minimal protection or rescue work," was misinterpreted by some to include actual protection training such as police dogs or Schutzhund dogs are trained to do in biting an attacker or biting a human on command. It is not clear how they could think that constituted "minimal" when actually assaulting and battering a person is far from minimal, but that's what happened. Other groups, such as the SDVA trained "threat" dogs or dogs that would growl and bark at people, which generated this response from the service dog community:
What did the DOJ mean by "minimal protection" in the first place? Back in 2001, I interviewed several people who were involved in the original discussions with the DOJ over these regulations and they indicated this phrase was in reference to protecting an individual during a seizure from injury from impacting dangerous surfaces or falling down stairs, and protection from pickpockets going through their belongings while they were incapacitated. At the time the predominate breed of choice was the Labrador Retriever so we're talking about dogs interfering with or blocking rather than threatening or biting in order to afford that "minimal protection" against pickpockets.
The DOJ published a guidance in 2002 which confirmed this was the DOJ's intended meaning of "minimal protection" when it said, "Service animals are animals that are individually trained to perform tasks for people with disabilities such as guiding people who are blind, alerting people who are deaf, pulling wheelchairs, alerting and protecting a person who is having a seizure, or performing other special tasks. Service animals are working animals, not pets." in the "Business Brief." (My emphasis) See: http://www.ada.gov/svcanimb.htm
When the Department revisited the regulatory language concerning the term "protection" starting in 2008, they received lots of feedback from the public expressing concerns about how people were interpreting "minimal protection" to include attack training, bite training, and threat training of service dogs. In their first Notice of Proposed Rulemaking (published in the Federal Register Volume 73, Number 117, Tuesday, June 17, 2008), the department related it's views on the issue of protection training in service dogs and protective service dogs.
"People have asserted, incorrectly, that use of such animals
is protected under the ADA. The Department reiterates that public
entities are not required to admit any animal that poses a direct
threat to the health or safety of others."
Consider a cane which is used as an assistive device for a person with a disability who requires a cane to walk. The ADA would generally require businesses to permit the use of the cane as an assistive device even though it is possible for a person to use the cane to assault or batter someone. The cane is not designed for for that purpose, ie to cause injury. It just happens that a person might use it, or a book, or shoe or some other ordinary object to commit a violence against another person. This situation changes drastically when the cane is the sort that has a sword concealed within it. At that point it is no longer just a cane, but becomes part assistive device and part weapon. Public entities and public accommodations may exclude the cane because of the weapon half of it's nature, even if they would ordinarily have to permit it based on it's assistive half.
Now consider a service dog. An ordinary service dog is conditioned to be very tolerant of the ordinary activities of humans and to be safe around people. It is an assistive device like the cane. But if you train the dog to do protection work, ie to bite or threaten on command, then it also becomes a weapon, just like the cane with the concealed sword.
While the ADA does not prevent a person from doing protection training with their service dog, it also does not protect their choice to do so, and businesses may legally exclude a protection trained dog from their premises even if it is also trained as a service dog, as a direct threat.
Some will attempt to argue that a dog cannot be a weapon, it's a dog. However, numerous courts across the US have in fact found that dogs with a proven ability and willingness to bite a human being either on command or as a part of the dog's own nature, can legally be considered weapons. The law makes no distinction based on whether the training to bite humans is done as part of a sport or for personal protection, only whether the dog has the natural propensity or the training to bite a human being or has a history of ever having bitten a human being for any reason. If the dog has been modified by training to bite on command or is temperamentally predisposed to biting a human, it is legally a weapon.
There are many legal cases addressing the issue of when a dog may be considered a weapon and the exact circumstances will vary by location within the use because different states and localities will have somewhat differently worded statutes concerning what constitutes a weapon. Here is one representative case if you are interested in researching this further on your own:
"We hold that, depending upon the circumstances of each case, a dog trained to attack humans on command, or one without training that follows such a command, and which is of sufficient size and strength relative to its victim to inflict death or great bodily injury, may be considered a "deadly weapon or instrument" within the meaning of section 245."