My experience has been the opposite; it is almost always up to the artist whether or not photography is allowed. For example, I saw "Prog Nation" in Tampa...for the first two bands, no photography was allowed...for the last two (Opeth and Dream Theater) it was OK. The venue had no opinion about it - but the security guards knew which bands had OK'd photography and which had said "no".
I know some venues have their own policies, but it my experience it's more often up to the artist.
As far as next month's upcoming Opeth show, I already contacted the venue, they said it's up to the bands and they won't know until the day of the show. So I'm interested in hearing from people who've actually seen Opeth recently and can say whether or not the band allowed photography at those shows.
Thanks -