They don’t want any evidence that the Union committed war crimes by mistreating POW’s on that site.
Then they better dig up the Confederate dead that died at Elmira Prison (NY), and are buried in Woodlawn Cemetery there, as well as taking down the monument erected in their memory in 1937 by the United Daughters of the Confederacy. Woodlawn wasn't a National Cemetery at the time they were buried, but became one in 1877.