In places where you could reasonably grab an actual decent shot of the band on stage, and not the bull***t camera phone from the back of the stadium, would allow you to take photos.
Outdoor festival type concerts are typically always fair game no matter the size.
Places that dont allow cameras at all are typically not places where you could even use it effectively. I really find it humorous when a place DOES let a nice dSLR in and the kid is 30 rows back with a prime lens. Nothing your 12mp camera cant crop for effect, right!?
So, as others have said, its the venue (and maybe half-heartedly the band itself) who has the final say in photography of the show.
I think the size of the venue vs the ability to take photos is on a logarithmic scale where the bigger the place the less chance you have to take photos.
Under 200 people, definitely take photos...
Over 2000 people, maybe take photos...
Over 5000 people, would you want to take photos?