I guess it is about the responsibility that you are keeping something with you, just like plants as well as animals. It is classified as illigal because we have people who just wanna keep them for the sake of fun. And we also have people who kill them in mass number what we can't have a definitive answers.
However, when you talk about conservation, you talk about studying about what is around us and understand them, with such point in heart, I guess it is a calculated risk that we have to take. Just like those corals are fishes you are keeping.. If you are not careful, they will die and the cycle repeats untils we are loosing a substantial number that our conservationist have to appeal to ban them from being kept as pets.
Just look at the "Nemo" for example and you will appreciate what I am talking about.
Just by asking yourself a good question - How sure are you, when your purchased corals or fishes are gonna live till their liefspan is up?
I guess the point is that we must be responsible to keep something by providing an enviroment that is enough for it feel as comfortable as natural enviroment, using the most advanced technology and eqiupment and investment - which all sum up to be a calculated risk that you must take, as you will never be sure till it is fullfilled.