i never completely understood the reasoning, and have heard several different reasons. Also NO FLASH is a major staple in the rule book.
The artist and management may feel that we are obstructing the view of the people in the front row ( i can understand that)
It distracts the artist while performing (aren't they being watched anyway)
Limit the number of photos in circulation (Lou Reed wouldn't allow any photos at his show)
Some artists have photographers that go on tour with them, and are on a salary. I'd assume that if you have a photo pass, its a definite privilege, and one shouldn't complain. However we were promised 3 songs, and at the last minute it was cut down to 2 songs. I think the artist would want to get paid if we got to photograph the entire show, since it becomes similar to a photoshoot at that point. I think the artist and management figure that press is needed, so they will allow just enough for media to do a story, but cut it off before the real show begins(ripping off shirts, crowed invasions, etc..) The 2 songs went by very fast, and me being a fan, it was even faster and nerve racking to say the least. All in all a great experience!