I can assure you, from long years of experience, that no one
can "make a good enough copy of some valuable artifact to publish it
(and thus make lots of money)..." with or without paying the museum
its normal photo and/or reproduction fees.  Believe me, I tried for
twenty years and never succeeded!
        Disruption of the viewing experience for other ,useum patrons
is a far more valid reason to restrict flash photography.
        Mike Lewis