Barthes claims news photographs and pornography cannot possess punctum. Why not?
To answer your question, one needs to understand Barthes definition of punctum. Punctum, a Latin word indicating a puncture wound or grammatical punctuation, is an inherent quality of a photograph. It is something that the photographer captures through his sense of art and observation, through what Barthes calls the photographer's "second sight" (Camera Lucida, Barthes). Punctum cannot be arranged by the photographer--it occurs before the photographer's eyes and they have the foresight (or second sight) to capture it. Often, punctum juxtaposes elements that are discontinuous with each other: out of continuity; from separate spheres; living opposing lives; having disparate experiences in a shared space.
An example of discontinuance is the famous Koen Wessing photograph showing two nuns passing behind three soldiers carrying weapons--the second nun is looking out toward the soldiers with stiffened features (perhaps in repugnance or dread). There is a discontinuance between the world of the nuns and that of the soldiers; between the soldiers focus and feelings and that of the nuns; between what the nuns are on their to attend to (ministering of some sort) and what the soldiers are attending to (military protection of some sort).
The nuns puncture the quietude of the observer's experience with the photograph. They send a small wound to the observer's psychological and emotional experience and perception. The nuns punctuate the photograph and thereby divert attention away from a simple chronicle of a moment and toward a special and unique occurrence frozen in time. Punctum is that inherent quality in the photograph that is neither planned nor arranged and that arrests the observer's experience with the photograph.
News photographs chronicle events and thus are said not to contain punctum. While this is generally true, it may be argued that it might occur that a photographer who is chronicling a news event may, through photographer's second sight, wittingly or even unwittingly capture a moment punctuated by punctum. Therefore it may be true that while the general rule is chronicles of news events do not contain punctum, the exception to the rule may be that news photographers may indeed capture discontinuous moments exposing punctum.
Pornography cannot contain punctum because pornographic photographs are carefully arranged by the photographer--and models are carefully posed, lighted, made up--to deliberately capture the most dramatized erotic images. It may be argued, however, that someone taking covert pornographic photographs may inadvertently hit upon a discontinuous moment and thus inadvertently capture a moment of punctum. Nonetheless, due to the undirected, unplanned, un-prearranged nature of photographs that capture discontinuance resulting in punctum--photographs that are a product depending upon second sight--it may be generally stated that news photographs, which chronicle, and pornographic photographs, which are carefully arranged for a heightened effect, do not possess Barthes concept of punctum.