I do get it.

We have a health guarantee that says you can exchange a sick animal within a week. It is not the pet store's responsibility to pay your vet bills for you.

Expecting a store to clip the wings of all of their birds is ridiculous, and if we ever get a hurt mouse returned to us we put it in the feeder bin. It's sad, but that's what happens when you combine business with live animals. That's why I (a pet store employee no less) spend much of my time at work trying to convince people to go to breeders rather than pet stores. It's just a better idea, and it discourages mass breeding of rodentia to supply pet sotres.

They're not necessarily being inhumane. Any pet store is occasionally going to get sick animals in, and every now and again they're bound to sell one to someone by mistake. People make mistakes, that's all there is to it.