Yeah, I guess they would have. what if there were conditions to this? Like maybe just waiting for the right time when all of the pieces were in place. because having the avatar locked up for a number of years would only give them time to train and escape. They would probably find out that they are the Avatar while kidnapped. This way, he wouldn't have time to learn to work with the elements in wherever they kept him, and would only be able to use his native element.

Does this make any sense? It doesn't feel like it.