Slater left his camera behind by mistake. The photos taken by Naruto were a pleasant surprise.
Photograph ownership is often determined by camera control. If you find my camera and take pictures, the camera is still mine but the copyrights in the photos belong to you. If I am your employer, on the other hand, and your employment duties include taking photos, the copyrights (generally) would belong to me even if you are the photographer.
The issue with Naruto, however, was not one of photographer. It was agreed that Naruto was the photographer. He took the pictures. The issue was standing - PETA's standing to sue on behalf of Naruto and Naruto's standing to sue for copyright infringement.
From the case: "The panel held that the complaint included facts sufficient to establish Article III standing because it alleged that the monkey was the author and owner of the photographs and had suffered concrete and particularized economic harms. The panel concluded that the monkey's Article III standing was not dependent on the sufficiency of PETAs, Inc., as a guardian or 'next friend.' The panel held that the monkey lacked statutory standing because the Copyright Act does not expressly authorize animals to file copyright infringement suits."
I liked the partial dissent which said: "We have no idea whether animals or objects wish to own copyrights or open bank accounts to hold their royalties from sales of pictures."
That sort of says it all.
I am pretty sure Naruto is thinking of things far removed from the photos he took of himself.