Actually, the only situation when police really don't do anything, is when you come to them with petty crimes/misdemeanors and are unable of right away identify, or provide something to easily identify the offender. I've rarely seen the police not acting when evidences of identity are provided. Like in this case. If they didn't react... it's probably because the victim wasn't European... so taking on this case would be complicated and probably end in nothing. Because most often, when the victims are strangers the case tend to end in the trash after a while. And policemen don't like the idea of doing work for nothing. But they should do it. And act more often than not.
Note : Actually the Police may be doing something, but he OP don't know it. Because what's true about French policemen, is that they're horrible at communicating.