The point is that if you go on a national battlefield it is illegal. You will be arrested and will lose your detector and possibly your vehicle. Fines go as high as $25000 and you could also get a few years in the pen. if you get caught more than once. These people don't mess around and the courts in these areas know all about relic hunters and will make an example out of you if caught.
Now, if you can get on some private areas which is doubtful around National Parks because so many people trespass there is absolutely nothing to be scared about. There are tens of thousands of unexploded shells out there, but they are reasonably safe if you are careful with them. If you mess with them to much they will kill you, like one killed a guy last year (2008) and severely injured another guy in 2007. Gunpowder is safe when wet, but when it dries out it is very dangerous. If a shell is cracked open keep it in water. If it is not cracked open and the fuse is still in the shell be careful. If it is a Borman fuse and it is not punched, be real careful.