Went to Beas River for the Olympics today, which was great fun.
Anyway, as we have never been at an equestrian event, we (and many others) brought an umbrella along, which became handy as it was drizzling on and off. Simple ponchos were actually provided at the venue, but we felt that with the rain being inconsistent, and the poncho creating an unbearable green house effect, it would be easier to use the umbrella when necessary.
The venue was huge, visitors were spread out nicely over the entire course, and we made sure not to block the view of other spectators when we used the umbrella. This worked well for about 30 minutes, and many other people did the same.
After standing for a while at one quiet spot, one of the volunteers suddenly asked us not to use the umbrella. As he walked away immediately, we were not able to ask for a reason but complied, and thought we were probably blocking somebody's view unknowingly.
15 minutes later we were walking to another part of the venue, again with umbrella. This time a seemingly more senior western official demanded rather rudely that we not use the umbrella. I asked him why, and he said it is not allowed as it would scare the horses. I pointing out to him that many people were using it and that I was not aware of this rule, but he was not impressed.
This time we got the ponchos out, but while walking to the other end of the venue, we noticed even officials using umbrellas, right next to the track! Even Princess Anne and her entourage were there with umbrellas and not with rain ponchos (I just saw it on TV...)!
Double-standards or plain oversight
I checked the official website and the umbrella rule is not listed under "Etiquette When Watching Equestrian Competitions" or "Special Notes", where other such rules are listed (no flash photography, no sudden movements, etc).
I finally found it under "Service for Spectators" right under Baby Stroller Storage, the reason given is "for your own safety and for consideration of others".
There was no sign at the entrance, and none of the hundreds of other volunteers pointed this out to us and others. (there were volunteers and officials every 20m at least...).
Another strange thing: They recommended to drink plenty of water, and when you arrive they tell you no drinks can be brought inside. At least they provided a free bottle of water once you were inside, but telling people beforehand would reduce waste (people had to throw away tons of drinks before entering the venue)