"Droit à l'image" covers use, duplication and distribution of pictures of people, and only if said people can be identified on the picture. I don't know about any french law that forbids actually taking pictures of people in a public place. In a private area, there's Code Pénal, Art. 226-1, and even then, the law says that unless they explicitly disagree, their consent is assumed. Once the shot is taken, you don't have the right to publish the image without their explicit consent, and if no consent of publication is given, you can publish the image as long as people cannot be identified.
(FWIW one can take and publish pictures of goods to one's heart's content, provided it causes no harm)
> If you try to take pictures of strangers in the subway you will be heckled and possibly assaulted, and the police will do nothing to stop it.
There is no legitimate reason for a nearby police member not to try to stop someone physically assaulting someone else, whatever the reason of the assault may be. Their duty is to at least inquire into the situation.