I'm not sure what labels you're looking at, but I believe Hikari and NLS both have some type of animal meal listed as their first ingredient. And if we're going to bring up the whole topic on fish aren't meant to eat pellets thing, I can always pull out the "well fish aren't meant to be kept in glass boxes but we do it anyway" LOL. In the wild, the likeliness of a fish obtaining a mammal or bird is pretty slim. Most fish scavenge detritus and plant matter.. And plus, I don't think a fresh water fish would ever be able to eat oysters.. or shrimp.. or spinach.. You know.. the list goes on and on lol.
Freshwater puffers do eat freshwater clams,mussels and shrimp. Clown loaches eat snails, mollusk tissue is pretty similar across the board be it fresh or marine. The aim here is to give the fish a broad spectrum of whole foods. If we were to apply your logic to people food we would be eating our meals pre-prepared by McDonald's and told that it's all we need and that to eat fresh organic whole foods is not good for us in the long term.