Ward, Wieland, and Cialdini:
When I visited the petrified forest as a child, it was fairly obvious to me why it was prohibited to pick up pieces of the petrified wood - samples in pockets would wander off, and the whole monument would slowly disappear.
On that same summer road trip, I logically wondered if by taking samples from the Grand Canyon I would be a) degrading the natural wonder by removing some of it, or b) adding to its beauty, since the canyon exists precisely because material has been removed.
A few days ago we had a visitor here from the National Parks Service, and she related a story about the National Cycad Monument in South Dakota that reminded me of all this - the Cycad Monument was decommissioned in 1957 because there were no fossils left to see. The fossils had all walked off in visiting pockets.
The Park Service's best estimate for current losses from the Petrified Forest are about 25,000 pounds (!) a year. People are caught every day (some with hundreds of pounds stuffed under their car seats). The forest is such an attractive site for theft that it has been used as a test site for studying what kinds of warnings are most effective at discouraging theft. The best estimate is that about 3% of the visitors actually steal material.
When I first visited Hawaii in 1985 with Caltech's Bob Sharp as a tour guide, he regaled us with tales of "Pele's curse," whereby if anyone took a piece of the Goddess Pele home (a piece of Hawaiian pumice, basalt, green sand, etc. etc.), she would bring bad luck upon them. In the Bishop Museum and the Hawai'i Volcanoes National Park Museum there are plently of examples of tourists who mailed back their 'souvenirs' (even their sandy shoes, in one case), thinking that their streak of bad luck after their holiday was due to Pele's curse.
In any case, my long-winded point is that this type of "curse myth" seems to be popping up at other parks - the Petrified Forest is getting souvenirs returned as well.
Perhaps there should be a study done about the effectiveness of public announcements saying "if you take anything except pictures, you will have really bad luck."
And yes, I do have a piece of Pele in my house. It's not illegal to take pieces of lava from Hawai'i. It's a really nice chunk of frothy green pumice and some Pele's hair that I collected from the side of Chain of Craters road. And yes, I have had bad luck since then. Only I am quite sure that one thing has absolutely nothing to do with the other. And did I take a piece of the Petrified Forest or the Grand Canyon? I'm not sure. It was so long ago that I can't honestly remember - I do remember being very tempted, and if I did, I have since lost the pieces, which is probably the sad truth about a fair bit of the ton-a-month rate for the Petrified Forest.