Well, I guess they were bored:

probably no children to molest or enough women to set up brothels.
Spectacular prehistoric depictions of animal and human figures created up to 6,000 years ago on Western Saharan rocks have been vandalised by United Nations peacekeepers, The Times has learnt.

Archaeological sites boasting ancient paintings and engravings of giraffes, buffalo and elephants have been defaced within the past two years by personnel attached to the UN mission, known by its French acronym, Minurso.

Graffiti, some of it more than a metre high and sprayed with paint meant for use for marking routes, now blights the rock art at Lajuad, an isolated site known as Devil Mountain, which is regarded by the local Sahrawi population as a mystical place of great cultural significance.
Used to be, anyway.

Julian J. Harston, the UN’s representative of the Secretary-General for Western Sahara and head of Minurso, said that he had been shocked by the scale of the vandalism. After visiting two of the sites, including Devil Mountain, this week, he said: “I was appalled. You’d think some of them would know better. These are officers, not squaddies.” The UN would take action against any officers “kind enough to leave their calling card. We will report it to the troop-contributing countries. We can move them.”
That's punishment, all right!

...Minurso staff have felt entitled to destroy elements of Western Sahara’s and the Sahrawis’ cultural heritage, despite being aware of UN ethics in peacekeeping, and in breach of legislation enshrined in the 1954 Convention for the Protection of Cultural Property in the Event of Armed Conflict.”
Them Conventions really got teeth, don't they?
It concludes: “Minurso personnel have played a major role in damaging archaeological sites, and such staff are engaged in the systematic defacement of valuable archaeological sites over a large area . . . the recent damage at Lajuad is unprecedented.”

The vandalism will reignite the debate about the conduct of UN peacekeepers after a series of scandals. Last January the UN admitted that more than 200 of its troops had been disciplined for sex offences, including rape and child abuse, in the preceding three years; in May it emerged that Paki-stani peacekeepers had been trading weapons with Congolese militia.

UN Peacekeepers: Defacing History That Others Won't!

Added: I was thinking about the line as a mystical place of great cultural significance. I wonder if there's some reason they couldn't say 'as a holy place.'