Nikon and Canon both have a feature where the camera digitally signs each image, so that you can "prove" that they weren't modified after the fact. Unfortunately, I believe someone managed to extract the signing key from a Nikon camera and there was some other vulnerability with a Canon.
It's signing the images, not encrypting them. This isn't attempting to deny you the ability to see the pictures. Instead, it's supposed to demonstrate that an image wasn't modified after acquisition. I think it essentially takes the pixels, calculates a signature, and then stashes it somewhere in the jpeg/raw header.
I can imagine some non-evil uses for that, like crime scene or insurance photographs. Heck, it might even be useful for online selling (here's an unretouched photo--look, no scratches!)
You're right that it's tricky to protect the signing key from a-user-who-is-also-an-adversary though.