Two photographers stand in the same place with the same lens at a demonstration. One frames so that the line of police is just outside the right edge; the other includes it. Why is that a claim about the event rather than a matter of taste?

B. The edge decides what is present; excluding police makes the crowd the whole situation.

Because the edge fixes the limit of the evidence. A viewer of the tighter frame cannot know the police are there, so the picture asserts a crowd without a confrontation. Neither frame lies about the light, and both are complete photographs, which is exactly why the choice of edge is an editorial act rather than a preference.