People have a problem with killing things they empathize with. Most people (Westerners anyway) empathize with cats and dogs because everyone had one growing up. It doesn't hurt that they're mammals like us either, There's nothing special about them that doesn't apply to rats and rabbits. Anyone who cant acknowledge that just cant see their own bias.
That said, I wouldn't kill a cat or a dog to feed a snake. I would probably feed dead cats or dogs to snakes provided they weren't killed for that purpose. I have no problem with others using cats or dogs as feeders. I'm just biased. I have known too many cats and dogs far too well to do that.
My huge (23lb) male tabby cat died just yesterday.
I would no more consider using his body as a food than I would consider using any member of my family as food.
Reptiles as feeders? why not.