This doesn't cause the fish any pain, unlike dyeing or tattooing. The genetic material from the jellyfish is added to the danio in its embryonic stage. The flourescence is passed down to the fish's offspring; it becomes part of the fish's DNA. This is a completely different process than dyeing or tattooing, and IMHO it isn't cruel to the fish (without getting into a discussion of the ethics of genetically altered animals).
Add blueberry oscars, "fruit" tetras, painted glassfish, and even some species of loaches to the list. Dyeing fish is a horrible process that results in massive deaths initially, stunted growth, susceptibility to disease, and an overall shortened lifespan in many cases.