Well from some point in time all fish originated from the wild. We can't learn how to tank raise a species of fish, without first taking them from the wild and putting them in a tank. If you argue that fish keeping is cruel(I agree with you that some parts of it are), what about fish that live in the wild. The don't have perfect lives either, tons of predators, pollution, and they can suffer from droughts and such. I have wild caught fish and I don't feel cruel for having them taken out of there homes, I doubt they care. Since they are healthy and spoiled rotten.
I would say that for the informed fish keeper, fish keeping is not cruel. Most cruelty comes from misinformed people, it is the same with any animals people want to keep in captivity.
Fish shops are there to house the fish temporarily, a young oscar doesn't need a giant tank in a LPS. Because it is not likely to stay there for very long. I don't see any cruelty in this, as long as the fish are kept healthy and buyers are informed correctly about the fishes needs. Also I feel it is the buyers responsibility to get reliable information, relying on someone else to inform you on how to care for a pet is IMO negligence. LPS carry books on fish and the internet is full of information.