Not every dog can become a service dog. It takes a great deal of training for a dog even to be considered for the title of "Service Dog," and many people not in the dog industry don't know that. My childhood dog had to be trained and tested before becoming a therapy dog, and all therapy dogs do is let people pet them! They think that just because their dog does some service for them (giving emotional support, fetching the paper, etc.) qualifies them to wear a service dog vest in public. That might sound pretty stupid, but so are a lot of people. Most people who are smart enough to sneak a normal dog on an airplane use the phony claim that they're an "Emotional Support Dog" and use that vest because that has a much less stringent set of requirements for qualification. Unfortunately, the teenage girl at the center of this story isn't that discreet.
This image is for illustration only, and the subjects are models; the image does not depict the actual subjects of the story.