I believe the investigation is because the guide took them to an location that were not authorised to so and not a normal off piste area.
I think it's from the perspective of the guide not being authorised rather than the area being forbidden but I'm not sure of this. In Europe things are different from what I understand about North American mountains in so far as there aren't out of bounds areas, generally you can go anywhere on the mountain in between the pistes etc. Sometimes runs are closed if there is a danger and marked as such. Obviously it's in you interest to respect that but you don't have patrollers enforcing it as such.
It also goes to show that an airbag does not completely mitigate all risk.
Whatever the circumstances its always very sad to see people being lost and I feel for all involved. The only way to take something positive is to let it be a reminder us all to not take chances yourself and understand that there's always still risk (which I'm sure most if not all of you are well aware).