That's because they aren't fuzzy and don't have facial expressions we like to think we can read. In the words of Jack Handy "If trees could scream, would we be so cavalier about cutting them down? We might, if they screamed all the time, for no good reason."
I think we should all try and take good care of living things, rather than assume living things we can't "read" simply don't feel anything. That's arrogance on ignorance. That said, given that all living things eventually die, what is an "appropriate" way for a fish or snail or invert to die? Of old age? How many die that way in the wild? A decent life is more important than mode of death, IMO.
There are plenty of people who would not care one bit if somebody sliced up a fish for No reason, but would throw a hissy fit and protest if they slaughtered a dolphin. I think it has nothing to do with "fuzzieness" or showin emotion or anything of tht sort.
I was taught that fish felt fear for their lives, and that is as far as feelings go. Explain why you can touch a fish out of water quickly and abruptly and it will flob everywhere, but with a steady hand you can shove aRusty hook slowly and carefully through any body part and it won't flinch a muscle.