I have seen the signs banning the use of metal detectors in or around Civil War sites. For instance, there are signs banning the use of metal detectors on Loudoun Heights, on the WV/VA border along the Appalachian Trail and this is an unimproved area just out in the woods, not at all like Gettysburg. Geocaching could be banned at these areas as well.
France and some other European countries either restrict or ban metal detector use throughout the entire country to prevent destruction of battle or other historic sites.
I think in a lot of European Countries it's allso a matter of safety for unexploded ordinance from WWII still around.