Cop wants to protect the privacy of someone else.
Some aspect of an investigation must remain secret or it will fail. For instance, video of a meeting between a regular cop and and undercover cop or informant could endanger the undercover cop or informant. OTOH, maybe such meetings should not occur in public?
A reasonable cop would be willing to cite his reason and clarify whether this is a request or a command with legal backing.
Have I overlooked something? Please put it in the comments.