Interesting. This ruling occurred months after the singing incident at the National Mall and was made by the D.C. circuit. So if this group had sung this month instead of two months ago, they wouldn't have been bothered and it would have been a non-issue. I guess it is too recent of a ruling to have been updated on the National Park Service website.
THis assumes that Federal Employees that confront these folks are up on the law.
We have dozens of examples of their ignorance of the law.