I'd really like to see evidence of such a law. I think that the reason we don't see much of the Grand Ducal Family in the media is
1] Luxembourg is a tiny, tiny, tiny country
2] They don't do much that's interesting to the media, who have nine other monarchies to keep up with.
I'm pretty sure Louis' girlfriend's pregnancy is the most interesting thing the media has had to dish on in years.
Also, there were photos published in Se og Hør of Guillaume walking around with Pia when he was still dating her in the town that Durham University is in, and the magazine's photographers were obviously not given permission by Guillaume or the Palace to take and publish these photos, and I don't recall the Court doing or saying anything about them.
Can someone provide us with specific documentation of this "hands off from the media" rule in Luxembourg? Otherwise it's just speculation.