I'm pretty sure nobody told you to return the animal.
This isn't strictly true, Petco expects to take a loss on live animals. They offer them to sell supplies and to encourage multi-pet families to choose Petco as the place to buy their dog/cat food and supplies.
Not that you should buy reptiles from them, you shouldn't. They are mostly wild-caught, and kept in sub-optimal conditions (I don't know if their panthers are wild caught or not). One purchase isn't really going to impact the way they stock their cages. Their goal is to keep the habitats full of juvenile specimens. If they die off, they will buy more to fill. If they get too large, they will sell them at a steep discount and buy more to fill.
The stores themselves have very little choice about which animals they buy and how many they should have. It is almost entirely about presentation. Locations tend to have a few evergreen species that they always carry (bearded dragons when in season, anoles, "Russian" tortoises, leopard geckos veiled chameleons...) and a few animals that coincide with some ad or promotion. If your local petco has a bunch of jackson's on sale with a PALS discount, they are most likely also selling Zoo-med chameleon kits at a discounted price. Stores occasionally (like when beardies are out of season, for instance) have 1 or 2 habitats where they are allowed to choose what they carry, but that is really the only time where an actual person might make the mental calculus of "oh, that species doesn't sell well here..."
I guess the short of it is, if you want to discourage Petco/smart from selling live reptiles, you shouldn't buy crickets, supplies, or your dog/cat food there. If those sides of the business are doing well, they will keep stocking their reptile habitats, regardless of how much money they waste on loss and veterinary care.