They were taken for 1 of 2 reasons:Lutz said:Presumably those that were taken off the road were confiscated because they were not insured, not because they were not taxed, or the owners were considered by the French authorities as being residents of France and not visitors,
1, They were not legal in their country of registration, i.e. no Tax or insurance or MoT, although frequently all 3. For a vehicle to be driven legally in an EU country it MUST be FULLY legal in it's country of registration.
2, Because they were French residents who hadn't re-registered or at least started the re-registration process within a month of their becoming resident.