Becuase "cap" guns as you call them are nothing like the smell, feel and effect of Black powder and would not allow correct demonstration of drill, should I also trade my yew longbow in for a plastic one? Showing the effect black powder has when fired is integral to demonstration at historical shows, have you ever seen 2 ranks of 30 fire by volley in real life? The field disappearing in a cloud of acrid smoke can not be recreated with "cap" guns, the blackened faces from smoke with riverlets of sweat you have at the end of large battle recreations can not be recreated with "cap" guns. Oddly what makes us safer when we are "charged" before and during battles is following the correct drills as laid down in the period, accidents occur when those drills are not followed.
Saying that in the 25 years I have been reenacting I've only ever seen 2 major accidents, one was black powder related when a rammer on a cannon crew forgot his drill and held his hand over the end of the rammer instead of around it -spark, bang and you can imagine the rest, needless to say he couldn't continue his career at Games Workshop HQ as a figure design sculptor. The other accident was in a situation where there was no black powder at all, a lad tripped over his own shield and fell mouth first onto his own (reenactment blunt) sword, luckily it was at an angle and the sword came out the side of his cheek rather than continue to the back of his throat.
You do hear of the odd incident at other events around the country and the world but they are few and far between. The thing you most commonly see is minor bone breaks from the ruff and tumble of battle or people collapsing with dehydration spending hot days in personal tin can saunas.
In the group I mostly work with now we have a 1 strike and you are no longer allowed to live fire rule where 'misplaced' firearms are concerned, many groups also apply this policy.
All that being said, if you do find yourself being held up by a scrote carrying a stolen musket duck for cover for the first shot and you then have at least 20 seconds while he tries to reload to nab the blighter