I have been trying to find out the actual legal reasoning.... I thought maybe the guillotine was listed in the U.S. Code, but it is not. It appears to fall under

Dangerous Weapon Broad Definition: Under District of Columbia and federal Capitol grounds laws, an object does not have to be a firearm to be deemed an illegal weapon. A large-blade execution device transported to a highly sensitive government zone is legally classified as an instrument capable of inflicting death or serious bodily injury, giving police the statutory authority to make the arrest.