There was a good NPR piece about this yesterday, which went into some detail about the disrepute the "detectorists" (hobbyists with metal detectors) are in, and how unfair that is given the close collaboration they've developed with museums over the last N years.
They aren't helpful if the dig into a new site disturbing the archeology to get at the 'treasure'. This was a ploughed field - it was about as disturbed as you can get.
If 1/2 goes to the farmer and 1/2 the guy with the metal detector, who is paying for the dig and what do they get out of it?