Heritage Action's text this morning on Treasure Ransoms ('How many British treasure payouts are far too high? All of them?' 4th June 2016) includes two juxtaposed photos the lower one concerning the effects and 'method' of artefact hunting:
I would suggest that the archaeological effects of the latter activity are unaffected by whether the artefact hunters had permission to be on that site at that time or not. In other words it is not the legality or otherwise of the activity which is the main issue here.