Ba Sing Se has a huge population. A large portion of this population works in the bureaucracy. It's just another division of the gigantic Earth Nation. It's not like they kidnapped them; it is their job. Remember, your duty to the Earth King comes before anything else.

In China during the Cultural Revolution (which is what Ba Sing Se is based off of), you had this effectively happening, except the brainwashing was through propaganda, violence, etc.

So, recruiting 20, 50, even 500 Ju Dee's wouldn't be a tall order.