Wildlife rehabbers absolutely do release wild ducks. The ducklings are raised appropriately and then released at the right age. Even non-wildlife rehabbers can raise wild ducklings for release. There's lots of information online about raising wild mallards and then releasing them, look it up 
This duckling is still a wild animal, with nothing that can prevent it from being released at the right age. It's natural instincts aren't going to go away just because it is in captivity. Why shouldn't it be able to have a natural life?