I understand this being declared a form of protest. Everyone in there would be compelled to stand still with their hand on their heart for the duration. Protesters would sing it all day thereby bringing government business to a halt. (Not saying that would be a bad thing)
I do not believe this is nefarious or any type of censorship.
Note: I just looked it up, and it has been illegal to sing the NA at the Lincoln Memorial since 2010. They said it “might offend” some visitors, and any singing must be “content neutral”. And here I was believing it went much deeper than that. Anyone offended by the singing of the NA can KMA. (I can do without all the urban yodeling that often accompanies it at sports events)
Face palm.