I think most of what I wanted to add to this thread has been said already, by people like nevertold and others, who speak from actual experience (and thank you for sharing, very brave of you).
All I can add is this: about 8 years ago, I volunteered in a Women's Refuge. Us volunteers often looked after the children in the on-site playroom, or brought them to the local library / Mc Donald's / playground, when their mothers were having their counselling sessions etc. We had to be so, so careful about what we said in public (eg, when returning to the centre, gathering up the children, we were only aloud to say 'lets go back to the playroom') - to the children, to each other, and to the women staying with us when we sometimes brought them shopping etc.
The fact was, some of the abusive exes managed to find out what refuge their ex and child/ren were staying in, and had someone linger nearby, waiting for them to leave to follow them and hopefully glean information about their life, or to harass and intimidate them. The women were secure in the refuge, but if for example we were overheard in public discussing where the woman's next counselling session was / who her solicitor was / where she was relocating to after she left the refuge etc, this could have been very dangerous. It may seem unlikely, but these rules were in place for a reason - because this had happened in the past. This was before the days of FB, so it frightens me to think how much easier it must be for abusive exes to stalk and intimidate now.
Okay, yes the number of vulnerable children in any one class, school, community even is likely to be very small. However, if schools feel the need to put these measures in place, clearly there is at least one child needing protection. If not allowing photography at such events keeps even one child safe, isn't it worth it? If your child was the vulnerable, at-risk one, wouldn't you think it was worth it?