A term I heard several years ago from an archaeologist friend was "pothunter." A pothunter is an untrained hobbyist who either doesn't care or doesn't know how to excavate a site carefully so as to get the maximum amount of information from it. His only interest is to find artifacts (usually pots) he can sell or add to his collection. These people often ruin a site for archaeologists. That's why archaeologists are usually very secretive about their sites – unless the sites can be kept under constant guard, which is seldom the case – until their work is finished.

Here's an article that illustrates what I'm talking about: Click here.

I suppose many pothunters use metal detectors (assuming the sites they're looking for are post-stone age), and that's why they have a bad reputation.