well they do grow them specifically for that purpose. the tree wouldn't even exist if someone wasn't farming it to be cut down and looked at in december. even the trees that don't get bought, i'm not sure that they get used for a better purpose or left to live. i suspect a lot of them just get cut down and tossed aside to make room for more trees to be planted.
Marijuana Christmas Tree Busted in Germany http://www.drugfreehomes.org/2010/12/marijuana-christmas-tree-busted-in-germany.html