Apparently dancing is considered a form of demonstration in the eyes of the law. This has happened before. These were 20 folks dancing quietly at around 12:00am on a Saturday morning with no one around.
http://www.myfoxdc.com/dpp/news/dc/jefferson-memorial-dance-suit-dismissed-012710A judge has dismissed a lawsuit filed by a woman who was arrested in April 2008 for dancing at the Jefferson Memorial.
U.S. District Judge John Bates stated in the court’s decision that the Park Service prohibits all demonstrations in the interior of the Jefferson Memorial, in order to maintain an "an atmosphere of calm, tranquility, and reverence,"
The lawsuit stemmed from an incident with Mary Oberwetter and friends at the Jefferson Memorial on the evening of April 12, 2008.
U.S. Park Police officers approached Oberwetter and others who were listening to music on headphones and dancing, and told them to stop.
Oberwetter refused to stop and was arrested. Charges were eventually dropped.
Oberwetter alleged that the arrest violated her right to free expression while the Park Service argued that the government has rights to regulate activities inside national memorials.