Far as i can tell, that is a Civil War re-enactor's website. Almost became one myself way back when, even picked up a 3-band Enfield musket but lose interest somewhere along the way.
Only thing that would make sense with the anti-MDing attitudes of the guys on that thread is that they are either amateur Historians, History teachers, Arch'ies, etc.
Their big beef appears to be only MDing on State land where CW battles or major skirmishes took place. I myself can half way see their point of view if thats the case.
Being a CW buff, i've been to many of the CW battlefield sites/re-enactments including Gettysburg. I can't imagine opening up the National Gettysburg Battlefield Park to metal detecting. IMO, it is indeed hallowed ground with likely many fallen still buried there in unmarked graves.
But on the other hand, i'm sure there are/has been archy digs going on there, hallowed ground or not. Whats good for the goose is not always good for the gander.
I'd think many rank&file CW re-enactors metal detect themselves for Civil War relics but on private property outside the main battlefield parks or encampments.