I wonder if you can get a note from the vet and bring that plus the bill back to PetCo, and try to get them to pay for the vet visit and/or give you a refund for the snake (which you obviously aren't going to return to their "care"). Not because it's necessarily a ton of money involved, but because they should have the incident in their records as something other than a problem-free sale or a presumably healthy animal that no one complained about.
I don't think you're wrong to rescue the snake by any means - that snake now has a shot at a decent life, which it would not have had otherwise. But PetCo shouldn't be rewarded for letting that happen.