It would seem to me that, as long as no human remains are found/disturbed, they really aren't hurting anything. Most of this stuff would simply turn to iron oxide in another fifty years or so anyway. With the amount of hardware lost, dropped, and discarded during battle I would think people in some areas could find some of this just planting a garden. I don't see what difference it really makes in who finds it. AS LONG AS NO HUMAN REMAINS ARE DISTURBED, much of this would simply rust to oblivion and be lost forever if it weren't dug out of the ground.
As for the 'black market' in war relics, if it weren't so taboo to look for them, the black market would dry up and largely disappear. I think the more of this type of stuff found, the better. Like I said, in a relatively short time it will mostly turn to rust. ALL of the above with the caveat, AS LONG AS NO REMAINS ARE DISTURBED.