As much as I know most people hate to hear it, I too work at a pet store (large retail chain that isn't Petsmart but by my contract, I'm not allowed to talk bad about them, so that's my obvious hint). It is COMPANY policy that snakes are to be fed pinkies once a week. If I were you, I'd take the feeding records and go straight to corporate with it. I'd also be absolutely sure to tell them that they were "feeding" crickets and let them know the outcome. I'm assuming that it wasn't acting as if it was in good health when you got it, and you should let them know. It is also against company policy to leave a sick animal on the sales floor and if it doesn't receive medical attention within 24 of being pulled off the sales floor, the manager is supposed to face a write up or possible loss of employment. Make sure they know you're upset and going to tell friends/families about it. It's still going to be a pet store, but if you can get the management in trouble (and trust me, they at least hear Every complaint that gets sent to corporate), it might scare them in to educating their employees enough so that the animals are receiving proper care.
Also, I'm so sorry to hear about that snake. It astonishes me how the small animal manager and store manager still have a job after such poor treatment to that little guy.