I can totally understand the disease part of it, but that's why they should be quarantined before being put into any water shared between the store's fish and the fish given up by the owner. And obviously that means more space, time, and money being dedicated to fish that aren't going to be making a profit for them, which usually means that store won't be willing to do so. But it would be nice if more small stores with employees who were really into the "hobby" (it's the easiest way to phrase it but I don't consider pets a hobby) could do adoptions on a small scale, only taking in a few at a time until the fish were given to new homes. My thoughts on this are probably unrealistic, I just naively assume that since I do what I can to go the extra mile for animals (and can't say no to those in need) that everyone else can do the same. :P
Kindness and respect should be shown to all animals, no matter if they are furry, scaly, big, or small. Be a hero to the animals and GO VEGAN!