The customers should be given some credit to be honest. Sure there's going to be some oddballs who rather 'play on their phone' than pay attention to the show, but it's very obvious that the majority of people are using camera's or phones at a live show to just capture one or two pics and maybe a short video.
I'm not necessarily opposed to the rule, but if DT were actually the ones to enforce this rule (which seems to NOT be the case here, I think it's being enforced by the venues), it would probably not be a vert smart move by them. You'd be surprised how much exposure bands get from pics and videos that people share online through facebook, instagram, reddit and the like.