From the sounds of it, the private key used to sign the photograph is IN the camera. So every user holds a private key in their hands they just can't easily get at to use to sign their slop.
And this means it just takes one smart fellow to tear the camera apart and extract the key, and they can sign al the pictures they want to, at least "from that camera".
I suppose as is the case with any asymmetric key-signing system, they'll have a "revocation list" they can use to burn any pictures signed by a "stolen" key once they discover it's been lifted and is out in the wild.
So at that point all the signature can say is "it's not AI until proven otherwise"
And lastly, what's to stop me from generating some slop and throwing it up on a nice monitor and taking a picture of it? Jackalopes are real, I can prove it, here's a certified picture I took of one!