that's because there is no such announcement. if anything, it's certain CMs taking it upon themselves. as i explain:
(1) if it was really important, there would be an announcement over the PA - people would probably do it anyway
(2) if it was really that important with people doing it anyway, they'd stop the whole thing altogether
(3) if it was really that important, someone like myself with the SLR and external flash and cables all over the place would probably be told something - i've never been told anything of the sort
(4) with everyone using flash regardless, how many incidents have there been?
the horses are trained to deal with it all. if a horse can't deal with it, the horse doesn't go on stage. there's plenty of worse things going on throughout the day for the horses that are pulling the carriages. at
disneyland, i was told one of the things that spooked the horses the most was the gun shot coming from the jungle cruise which you can clearly hear in town square there.