Yeah, but the reality is that if you allow photographs before the show then people will start arguing that their photography was OK because the show had only just started, and it's not disturbing anyone, and it was just them, and blah blah blah. People talk and use phones before the show. They also do those things during the show.
Whenever you create two states and a boundary you also create a whole bunch of people who automatically put everything they want to do on the "OK" side of the boundary and argue the toss on that basis. So people only go over the speed limit a little bit, or only overstay their parking ticket for a few minutes, or take just the one flash photograph in a museum, or send just one text message during a show, and they always have excuses: it's OK if it's just me, or I saw other people do it so I didn't realise it was wrong, or I thought it would be OK just this once, or this time it's really important, or I didn't break the rules by much.
Do you really think that if you allow photography at some times and not others then anyone who breaks the rules is going to quietly admit that they knew they were in the wrong?