Just a guess, but since it was a rock concert, maybe they were worried about "Date rape" drugs. The commonly used ones are odorless, tasteless and colorless. You slip some into a bottle of water and then offer it to a girl who didn't bring any, and then help her out to your car with the excuse that the heat must have gotten to her, then drive to your place and do what you want. A bar in Trenton had a problem with guys paying unscrupulous bartenders to slip the stuff into drinks when certain women ordered them. My Chief's secretary was one, but got rescued by her friends before the wrong guy could take advantage of her.

Anyway, not allowing anyone to bring their own water in is a band-aid approach, since you could easily conceal a tablet or two in y9ur clothes and add it to the water being sold at the concert.

It would have been better to warn people to bring their own water and not dring any offered to them by strangers.

And, to close, if someone at a concert offers you a bottle of water, give the bottle a good shake. The drugs will cause it to foam like detergent.