At my shelter, the only dogs/cats that get put down are either extremely contagious/deathly ill dogs/cats or animals involved in court cases. A court case animal is just that, they are currently in a court case, usually a mauling, and professional dog trainers care for them because they are aggressive. It is almost always a mandatory euthanization based on the findings of the court. When I say deathly ill, I mean things such as Distemper, Parvo, and Kennel Cough in dogs and Feline AIDS in cats, just to name a few.
Hopefully you're not misremembering, but they really shouldn't euthanize dogs for kennel cough if they're only doing it for deathly ill animals. Kennel cough is amazingly common, especially in shelter environments. The typical case can be handled with a course of antibiotics.