Well, I always assumed this was a universal phenomenon, but perhaps it's yet one more example of America's irresponsible disposable culture.....

People get animals as pets (dogs, cats, you name it). Then they move, or they get tired of their pet, or the pet pees on some furniture or what have you. And they abandon it. Or they have a litter of kittens/puppies and their owners abandon these. Sooner or later these abandoned animals find their way to the local shelters. Now, there's hard calculus of life at stake here. In order to take in new strays, the shelter will eventually have to make room by getting rid of some (or many) that don't get placed. Those animals that don't get placed after a certain period of time, or those that get deemed as unadoptable (due to abuse/neglect/illness/injury) usually get gassed right away.

Some heartless buggers actually take their now-unwanted pets down to the shelter themselves, comforting themselves with the lie that Sparky will have a new owner by nightfall.

This is why I hate pet-stores so badly. If you're looking for a Blue Tick Hound because you want to hunt or you want an Akita because you're getting into breeding, fine. But the vast majority of Americans just want a pet. Go to the damn shelter. And if really just have to have a pedigreed dog, go to a repubatable breeder. $300 for a purebred German Shepherd? Not possible, unless the animal is the result of a puppy-mill.