Disappearing is sudden government abduction with no paper trail. You criticize the government and then all a sudden, your colleagues and friends don't know what happened to you and can't contact you.
It's an important difference. Disappearing is more ominous and has a greater chilling effect. The government isn't putting you in a comfortable hotel, and they aren't killing you silently. They're torturing you for punishment and for information, and threatening your family. It's far more sinister.