Meh. I say that if they don't want pictures taken to say "please turn your flashes off" and/or say "I would prefer if you don't take pictures but if you must, please make sure the flash is off".
I went to a small venue show with my daughter a couple of weeks ago and I took a number of pictures with my phone. NO FLASH, mind you, but I did take probably 30+ pix during all three bands. Why? Not so that I can "prove that I was there" (that's just stupid), but because I enjoyed the show and I want a lasting memory of it.
When you go to your kids' plays or band concerts or anything like that, do you take pictures? When you go to a birthday party, do you take pictures? When you go on vacation, do you take pictures? Does ANYONE do it to prove that they were there? Well, surely there are some, but most of us do it to have a physical (or digital) replication of that memory that we can look back on and hopefully enjoy for many years to come, and also to pass on to friends, families and such.
Why should a concert setting be any different. If I spend $5 to see a show or I spend $300 to see a full-on concert, why the hell can't I take a picture (or 20) to retain that lasting memory?!
People questioning other people's intentions (or quality) with those pix or vids is pretty narrow-sighted if you ask me, based solely on the examples I gave above. We take pix and videos for many different reasons, why should a concert setting be any different? Why should some prima-donna a-hole music artist be able to tell us otherwise?
In an age where we can take and keep those types of things at our finger tips and in our pockets or purses, there is no way to stop it. So to the ahole artist that demands we don't do it, I suggest you use a different approach and ask politely (as Bush did here) but I would include that part about the flash. I do understand that they likely don't like the distraction of having flashes going off in their faces while trying to put on a good show. Flashless pix and video will NOT affect the artist in any way, shape or form.
And I do get Vehicle's point of view as well.... respect those around you.