Since snaps disappear seconds after they are opened, users feel comfortable sending spontaneous and personal messages that they would not want ingrained into digital histories. I donno but taking a screenshot and posting it on FB isn't very difficult. Do people really believe the images 'disappear'?
I think this gets to the tone of the article, as communicating this way implies a level of trust and/or intimacy usually reserved for analog methods.
Snapchat is brilliant in this regard. I think we're going to see a lot more communication platforms built on limitations rather than sprawling features.