Derek Fincham said it, I'm quoting him here verbatim, this is not some Third World country with 20% literacy rate, this is Central California (Derek Fincham, 'Losing California’s ancient petroglyphs', Illicit Cultural Property, August 26, 2014):
Central California PBS affiliate KVIE has a segment showing and discussing the theft and destruction of ancient petroglyphs from California. It shows some of the sites themselves, the damage they have suffered, and a good overview of the laws protecting these sites. The segment really hits its stride in pointing out the disconnect between laws protecting these sites, and the local populations. There is a lot more public awareness needed. People should know better, but they don’t yet, and cultural resource managers need to redouble their efforts to do a better job educating the public about why they shouldn’t damage sites and remove items.Which should go hand in hand with educating the US public about the damage done by the no-questions -asked antiquities market as a whole. Why is this not making much progress? Why do we find even US broadsheets writing uncritically and blithely of the collection of "ancient art" as though it was an envronmentally beneficial phenomenon, when the Americans have such destruction on their doorstep? Is it really so difficult to make the connection?
Watch Torn - Recovering California’s Stolen Cultural Heritage on PBS, it's really quite shocking.