If it's a US Park, then 1st Amendment trumps all...especially
in an educational or academic program. And these songs are
historical/academic material.

It's a vastly different situation than if this were a private
venue. It's not.

Every USPS employee holds up his or her hand and agrees to
uphold the US Constitution as a condition of employment.
Some of them think that procedure manuals and policy memos
trump the constitution.

Drop me a line on my personal email (greg@bullough.org) and
we'll discuss it more. I think I may have been down that very
road.