Even if that particular tree was inside of a steel cage, some people would have still tried to get to it. The J. Tree would have required a guard shack near it with 24 hour security.
http://www.ghosttownexplorers.org/california/U2/u2.htm
https://en.wikipedia.org/wiki/Joshua_Tree_National_Park
http://www.u2interference.com/forums/f285/rsoc-269-desert-sky-172327-3.html
http://www.nps.gov/jotr/index.htm , http://photos.calipidder.com/keyword/u2
http://bootlegpedia.com/en/product/G.R._045-046-047
http://hub.yourtakeonmusic.com/hub/u2bootlegs
https://en.wikipedia.org/wiki/The_Joshua_Tree#Packaging_and_title
