From the NPR article linked above

"Federal law prohibits political campaign or election-related activities within Army National Military Cemeteries, to include photographers, content creators or any other persons attending for purposes, or in direct support of a partisan political candidate's campaign," according to the statement. "Arlington National Cemetery reinforced and widely shared this law and its prohibitions with all participants."

I don't know the details of that, but presumably because it would be considered disrespectful.