I definitely understand where you're coming from Paty, and if you'd find a sympathetic ear anywhere, it would be here. I share your distaste for this treatment, and also find people's complete apathy appalling. "Just fish!" Ugh!
I was told of a fraternity that gets feeder fish in order to make the pledges think they have to eat them (a trust thing). They don't really make them eat them but I doubt they get returned or placed in a tank.
This begs the slippery slope question: surely nobody would do this with a dog, none of us (here on AA) would do it with a clown fish, or a cichlid, or a goldfish. Yet we do (I do at least) buy feeder fish for my Tex Cichlid as a treat every once in a while. Is this cruel? Some would say yes, myself included, I think it's pretty cruel for the feeder fish but do it anyway.
Some might argue it's more humane to give a fish a lightning quick ending on a frat house floor than to make it live in fear of a predator in an enclosed tank for days on end, only to eventually get eaten alive.
Mice and snakes are a great example, b/c mice have one up on fish in a lot of people's minds: They're mammals and they're furry.
Fur seems to have a lot to do with people's perceptions of animal cruelty doesn't it? Add fur and you add another level of sensitivity in most people's minds. Weird. What makes us feel the way we do about certain animals? Intelligence? Appearance? Demonstrative emotions?
I'm not going to lose sleep over killing some insects, but are they not entitled to humane treatment just as much as a puppy is?
My point is only that there is no one "right" answer. People's idiosyncracies towards how they feel towards animals vary widely.
I also think it's interesting how, if confronted between a choice to help a stray dog, or a stray human, the vast vast majority would help the dog. True, you can't take a bum home and make him all nice and cuddly, but interesting all the same.