True enough about plain old copper pipe. But when the lowlifes are bringing in entire air conditioners with sawed off connectors, or historical artifacts, or graveside urns, they can’t plausibly say “I had NO idea it was stolen”.
You're now bringing in a hypothetical argument here when there is no evidence this scrap yard ever dealt with "historical artifacts".
And furthermore, while there may be isolated instances of what you mentioned occuring, it is unlikely that the scrap metal industry is in the habit of purchasing such artifacts as scrap without notifying the proper authorities.