To use journalistic lingo, here's the story's nut graf, which gives the big picture:
When the landmark Americans with Disabilities Act (ADA) was signed into law by President George H.W. Bush in 1990, its creators certainly did not consider that their legislation -- which defines a service creature as "any animal individually trained to do work or perform tasks for the benefit of an individual with a disability" -- would pave the way toward allowing someone to walk an antisocial dog that had not received formal training into a library. In the nearly 20 ensuing years, the applicable definition of what constitutes a "disability" has expanded -- as has the very definition of a "service animal." In many cases, this has worked out to be a great thing. Agoraphobics and AIDS patients, for example, have been compelled to leave their homes and socialize because of the needs of their service creatures. Some homeless people have quit drinking and drugs to be able to properly care for their animals. But the ADA's amorphousness -- and its daunting penalties for those who deny the rights of the disabled -- has allowed the envelope to be pushed very far indeed.
In San Francisco, snakes, lizards, pit bulls, chickens, pigeons, and rodents have all been declared service animals, hauled onto public transportation, housed legally in city apartments, and, essentially, given the full run of the city.