The reality is that removal of one hatchling dragon will have no effect on the local population. Being a hatchling, it should settle in well to captivity if appropriately looked after. I personally don't have any ethical issues with wild collected animals being brought into captivity. So what are the issues?
There is a recognised need for regulation on the taking of animals from the wild. This stems from the harsh reality that many species have been extirpated or brought to the brink of extinction through unregulated collection for food, the pet trade, their skins and the like. As a consequence, it is illegal for an unlicensed person to take native animals from the wild. While this primarily exists to protect native animals from commercial over-exploitation, it applies to every individual as well. Queensland and Tasmania do recognise this and have set up their systems to allow limited wild taking. In SA it is illegal, as you know. That is your choice and your problem if caught and it does not worry me.
There is, however, another issue that does worry me and has been commented on. If you are going to keep any animal in captivity, then you have an obligation to learn the husbandry required so that the needs of the animal can be met. Otherwise you are likely to compromise its welfare. I must emphasise here, that whatever you have learned in the past with keeping reptiles does not automatically apply to all species. Each species has its own specific needs that must be met. Geckoman went to the effort to locate an authoritative caresheet you could use. Yet for some reason you apparently feel you don't need it. That is a serious issue for me.
Allow me to point out a couple of reason why you DO need to take the advice of those who know. Keeping a lizard alive for a week or two is no indication you are looking after it correctly. You could shove it in a shoe box under your bed for that time and it would still be alive. Use of aquaria as enclosure is extremely problematic and requires a few years of practical experience in order to set one up correctly for reptiles. I do not know what you do when placing it in the sun but that is fraught with potential problems. How often, for how long, at what ambient temperatures, supervised? Protected from arial predators or neighbour's cats etc etc. You put vegies in there for it to eat. They are exclusive insectivores. Do you know anything of their normal behaviour in nature? What they primarily eat? Do you know they construct a burrow at the base of a grass clump or hummock or small shrub. Are you away they make forays fro this to forage for food? Are you aware they quickly retire to their burrow when threatened or full up or too hot?... Do some reading and research your animal. For its benefit.
By your admission you have a very limited knowledge of reptiles. You said a gecko you had nuserd back to condition (tail = fat store) was eaten by ants, so obviously something you did did not work, because that is not normal. No-one laughed at you for confusing dragons and geckoes, a really basic error, because we knew where you where you were coming from. So for you to suggest that you know enough about husbandry, which is a lot more complex than simple recognition of reptile groups, is foolhardy to say the least. You may not mean it too, but it also comes across arrogant and lacking gratitude, so others will be understandably "not happy". the lizard is the loser in all this and that is why others feel so strongly about its release.
My advice would be to let the hatchling go. Get hold of the care sheet offered and do some background reading on C. pictus. Decide if you can meet its needs and are prepare to pay what it costs. I the answers are yes, you can then make your decision about how you intend to procure your lizard(s).
Blue