I think it's a bit more than that. If a random stranger on the street asked you to delete some pictures, then you'd take a judgement call. However a police officer APPEARS to have more authority (to the average member of the public) and a request from a police officer would be taken more as an instruction to most people.
I appreciate that their request has no more authority in law, but perception is everything, add in a little menace and veiled threats of arrest and lots of people would just follow what they'd perceive as an order from a law enforcement officer.