The fine in France can be up to ‚¬1500 and they can, at their discretion, CONFISCATE THE VEHICLE (sorry about shouting) but if the device cannot be confiscated 'in isolation' then they can (have have) seize your vehicle.
Also, from the AA website
"Effective from 3 January 2012 French laws prohibiting drivers from carrying devices capable of detecting speed cameras have been extended to include devices able to warn or inform of the location of speed cameras e.g. satnav or gps systems capable of showing speed camera sites as PoI."