I can see why breeders cull imperfect fish. Really, what you put out there and sell is a gauge as to the quality of your stock. There are some who feel it is a bad reflection on them and their breeding program to sell off low quality stock or 'culls', even as pets.
Some people simply have more stringent standards than others. It doesn't make them anymore right or wrong than those who choose not to cull any of their fish. As long as it is done humanely I have no issue with breeders choosing to cull stock they feel does not make the grade.
Ever since we first domesticated animals and started selectively breeding them, culling has never just been about removing those unhealthy individuals from the gene pool. Perfectly healthy animals that have just not fit the standard or have undesirable traits have also been culled.
While it seems cruel or unethical to some, honestly they are only fish. I have no issue with culling fish at any stage of life, as long as it is done as quickly and humanely as possible as I mentioned before.