There are a great many things which are, technically speaking, legal according to the local jurisdiction but illegal according to a larger overlapping jurisdiction. DC need not declare something illegal or legal, just leave enforcement up to federal agencies - which becomes interesting when the feds have an official non-enforcement policy.
What a strange legal place to be after US v Raich...
If there is one thing I cannot stand here is “federal enforcement” of laws. When a park police officer pulls me over for having a cell phone in my hand, I want to go to NPS headquarters and rip that place apart.