This is another one of these it depends sort of on the range requirements. There are some flint locks that either do not have a half cock notch in the tumbler or the half cock notch doesn't hold. Since the range officer has a lot of rifles to watch over for safety reasons, having the hammer down makes his job a bit easier since there is no way for the lock to cause an accidental discharge due to a fall from the half cock position when the lock is fully down. Also a glance will verify that all locks are in the same safe condition. Yes, I am aware that an ember may still be luring in the barrel, but the likelihood of such a discharge is unlikely. Almost as unlikely as a hammer falling from half cock with the pan closed. The hammer at half cock and pan open is a safe position. Hammer down, pan open just makes the range officer's job a bit easier.
I believe they think there is a chance that the cock could drop out of half cock and if say the Flint struck the side of the barrel could cause sparks setting off the load.
i refuse to go shoot at a range with peacocks like that strutting around causing a commotion. I have my own range and thats where i stick to unless i am at rendezvous. Some of these places push safety so much, that they are a safety issue themselves when it comes to those guys running around yelling, causing a shooter to become side tracked.Some range rules don’t have to make sense. But you have got to play by field rules anyway.
If you don't have, and I mean if you don't have common sense...stay away from any type of weapon, including a baseball bat....I passed my NMLRA range officer test several years ago and they do require cock down. What is to prevent frizzen from closing and coming into contact with the flint in the let down cock? Not likely to set it off but unless there is something mechanically wrong with the half cock, not likely that the cock will fall out of half cock and cause a spark. Some rules might be dreamed up by lawyers or clubs trying to avoid any future litigation. Case in point - blowing down the muzzle of a just fired muzzle loader. The explanation for this is that one of the ten commandments of firearm safety is to treat every firearm as if it were loaded. A novice seeing a shooter blowing down the muzzle might get the wrong idea that it is o.k. to put one's mouth over a "loaded" weapon.