From a Smithsonian press release:
For the safety of our visitors and collections, the Smithsonian prohibits the use of tripods or monopods in our museums and gardens. Effective today, March 3, monopod selfie sticks are included in this policy.
This is a preventive measure to protect visitors and objects, especially during crowded conditions.
We encourage museum visitors to take selfies and share their experiences—and leave the selfie sticks in their bags.
Good move and I guess it must’ve become a problem at other museums if not the Smithsonian yet.