PETA argued that the monkey should have copyright in the photo. That argument was rejected by the district court, and didn't look super promising in the appeals court oral argument.
There was a separate discussion about whether the photographer could have copyright in the photo, in particular the Wikimedia Foundation claimed he should not. That discussion has not been tried in court at all, and it seems far from certain (lots of legal commenters disagreed).
https://en.wikipedia.org/wiki/Monkey_selfie_copyright_disput...
A good example are music remixes, you need to actually contribute a lot yourself to have copyright.
If your camera gets stolen by someone, and they take a picture, without you having any influence in that, you do not own the picture. You can not force people who want to use it to pay.
A camera thieve shouldn't profit from a misdeed, and the picture shouldn't be in the public domain by default. So, the photo has to be deleted because noone can claim copyright?
You can not own copyright of something you didn't created, when someone — be it a toddler or an animal — steals your camera and takes a photo, you do not own it.
Now the real question is if the animal owns the photo, or if no one does.
But US law allows unintuitively that animals can own things, and manage them. As lots of rich people had the idea to set a pet as their heir in their will, precedent was made for that.
So the only question is if the court would follow the precedent, or follow the agencies' opinion that the image is public domain.
Which led to the settlement, which argues that 25% of the image belongs to the management of the habitat where the ape lives, and the rest to the photographer.
Because you don’t have any copyright to photos made with your camera, but only to photos made by you.