As a photographer, I do not use flash without prior permission, and do not stand in a place that is 'better' (closer or more central) than my seat. I use a digital that lets me see the screen so I can hold it up briefly to take a shot without needing to stand.
However, at an open outdoor event such as a festival, where no flash is required and no movement needed to get a good shot, I do not worry about it. My photos are generally for my own entertainment anyway.
On the other hand, if I get a good shot, I send it to the performer (and a number have wound up on CDs and other material of various performers), I do not sell photos of someone to another party, and do not post any I think are not flattering or accurate, preferably both; I rarely post a shot without the performer's express permission, though on the 'Cat I have sometimes posted festival pictures. I do not video anyone even at a festival without permission (unless I am the festival's official photographer). I heed 'no photography' or 'no flash photography' signs or announcements.
If performers accept that it is really not enforceable, what would performers like to see as guidelines or general rules of conduct with a camera?