I think what's going on is a kind of snobbery from people with the huge 500G + tanks against everyone else. We can keep the same large species of fish but since I only have a 240, I'm being "selfish" and "self-centered" and the guy with the 1000G tank is not, even though I might actually be taking better care of that fish.
If we used your reasoning, then small fish being taken out of rivers and put in aquaria is a selfish act too. Nothing can even come close to being like the conditions in the natural environment. But since a fishkeeper might have a huge mega tank, he feels like he is doing less wrong than someone with a smaller tank. That's not being less selfish; that's just being more self-righteous.
So fishing would be the ultimate selfish act then because you're killing the fish right? This selfish and "your 300G tank is not enough" argument can go on forever. I think people should be more reasonable about things though. Anytime you take a fish out of the wild, it's probably not going to reach the max size it would've gotten to in the wild. But no way does that mean someone who doesn't have a 10,000 G tank like johnptc can't take good care of monster fish.