'Humane' in euthanizing animals is defined as painless, quick and with a degree of dignity. I am afraid 'why' isn't really a part of it.
In some ways breeders of larger animals have it easier, a dog that isn't perfect can be de-sexed and sold to a family as a loving pet. To that degree for a fish breeder one could always make a deal with a local pet store or advertise 'pet only' Betta for private sale.
Other options people could look into is making a deal with people needing large amounts of fish for study (universities looking into treatment perhaps) could take the genetically inferior and look after them while they conduct tests. The fish gets an happy (if somewhat shorter) life and help their brother and sister Bettas the world over.
In some cases people could argue that keeping their genetic failures, might be good in the long term. I am put in mind of two cat breeds, the stumpie and the laperm. Both were cats that simply appeared out of nowhere, genetic mutations that were separated and bred to produce new true breeds. Even a mutt can shine.
[Edit] The stumpy is called a munchkin cat. Wrong breed name there.
Last edited by Taeanna; 04-05-2013 at 08:49 AM.
Reason: information update