This is really an ethical question so I don't think you're going to find a clear cut answer. If you put large tangs, or tangs that grow large in a 75g or less aquarium you're going to get remarks on the internet. This is everywhere. That said, I think there's something to be said for whether or not your fish are tank bred or wild caught, what particular species it is, what your rockscape is like, what it's tank mates are. You can do the restaurant style fish swap where you grow out the cute the young fish, but trade them back to the fish shop when they outgrow your aquarium. Having worked in a pet shop for 2 years, my experience is that these large fish are very difficult to sell. For every person that has a tank big enough for a full grown regal tang, there were another 100 who want the juveniles to keep until they grow up. If we could chuck the fish back in the ocean when we were done it wouldn't be a problem but that's not how it works.
For me personally, if you're dealing with fish that are bred in captivity this issue disappears unless you're vegetarian. There's the ecological concern of taking exotic animals from the wild which are not nearly as common as food fish. The least I can do is try and provide them a home where that most closely replicates a small section of their environment. If I am keeping fish that shoal together in the wild, I like to keep them in a tank big enough to replicate that at home. This is just personal opinion though, I wouldn't judge you for going a different path, but it's food for thought.