So after the concert, everyone has to go back through the turnstiles to get their bags unlocked and returned to the promoter? What if someone says they left their phone in the car so they don't need a bag - are they searched? Do people need to surrender something like a license to get a bag? If not, what prevents them from either leaving with the bag or ripping it open during the show, making the process much more expensive for the performer/promoter? What about people who can't bear to be out of contact with their kids caregiver even for the length of a show? I would guess that the bag makes vibration less effective, and they are unlikely to ever hear ringing at a concert. (I wouldn't have an issue with it, but don't respond that they should put up with it since it was common in my day - people's expectations have changed.)
I can understand this more for comedians - a leak of their material is much more important, since the difference between hearing their material on a smartphone or live is pretty small, so long as the audio is good enough to understand. However, the difference between being at a concert and watching someone's cell phone video of a concert is so large that I can't imagine anyone saying, "No, I don't need to go see Alicia (or whomever) - the cell phone video was just as good as being there."