A monkey didn't sneak into his hotel room and take a happy snap. He sought them out and spent days with them, and handed his equipment to them. That's not 'indirect causality'.
By your reasoning, if I was to put a camera in a tumble-dryer and let it snap photos while spinning, those photos would not be mine because whilst I set up the circumstances, the actual photos were not specifically composed by me.
Far too much emphasis is placed here on who pressed the physical button, and not who brought everything together.
I don't have a problem with that consequence. Copyright is supposed to reward creativity. I don't consider "photo made by putting camera into a tumble dryer" creative in any meaningful sense. I understand that a lot of modern performance art would probably be excluded with this approach - but I don't see it as a problem, either.
The macaques were simply fooling around with a plaything and had no comprehension of what a selfie is; there is no artistic intent on their part. On the other hand, the photographer by his account appeared to go into this project with the specific intent of obtaining a monkey selfie. The concept, the subjects and their moods, lighting, shot settings, etc. were all his doing, and by any reasonable yardstick his work should be recognized as a form of artistry, just as how it is not exclusively the cinematographer, but the Director as well who is entitled to copyright on a film.
And yet we have people arguing in court that by the legal specifics of who owns copyright on photographs, this is purely the monkey's "work" (and winning).