This is not work for hire; it's not any sort of contract arrangement. It's just a kind pro giving away services. I'm not any sort of lawyer, but it sure seems to be that, in handing the camera back to the owner and walking away, the photo-taker is almost literally handing away any rights to the photo.
From a practical standpoint, this is certainly true. From a legal standpoint this is much less clear.
Argh... Again, the Berne convention covers anonymous authors. You are deemed to represent him and empowered to make good on his rights («réputé répresenter l'auteur et fondé à faire valoir ses droits»).
IANAL but case closed. Right?
Daniel