YANBU to expect people to stick to the rules. If the sign says no photographs then everyone should respect the rules are there for a reason.
Did the woman taking the photographs realise that she was responsible for triggering your seizure?
I had a very mild form of epilepsy that was a problem when I was about eleven or so but gradually subsided in my later teens. It was controlled with medication for awhile but I haven't needed to take it for years. But as you say, many things can make the risk of having a seizure increase. Mine were always worse just before and during my period and at their worst they could be triggered by the smell of coffee, cigarettes or bleach or even by having someone raise their voice in anger.
The parents at your daughters concert were disgraceful and surely they realised that they were responsible for your seizure? It would have been a small enough event and hall wouldn't it?
But I can also see why people want to take photo's of events and days out too, although again, they should keep to the rules or respect situations like yours when they are made clear to them.
DS is our third child, following our sons stillbirth and our daughter dying shortly after she was born. We were not allowed to take any photos of our son and the hospital camera was not charged properly when our daughter was born (we were admitted in an emergency and didn't have our camera with us) so we have two very bad photo's taken on my phone while she was alive and a very small handful of photo's taken with a borrowed camera after she had died, perhaps ten different ones in total.
It's one of our biggest regrets. I wish we had more photo's of our daughter and even just one of our son. So when we had DS we went to the other extreme and everything gets photographed. I wouldn't take a photo where we were asked not to, but I like to have my camera with me wherever we go.
My DH is in the armed forces and works away in the week. He's also spent a lot of time abroad over the past nine months and will be out of the country for most of this year. Photo's of DS doing normal things as well as special ones make him feel like he's not completely missing everything.
I wouldn't put those reasons above a safety rule asking for no photos, or above general consideration for others such as in the middle of a play at the theatre or the last trip we made to the cinema when a whole row of teenagers were texting friends and taking photo's of each other on their phones. And I wouldn't put it above your health if I had been one of the parents at the Brownie group.
But in a normal situation, even the old fashioned sweet shop one, I might have taken a photo or two of DS if there were no signs saying not to and not because of Facebook but because we love having the photo's and realise how precious they are. DS loves looking through the albums and they are a great way of making and keeping memories for him.