Now that all the fun stuff was said, to present the range side. Most of the Black powder cannons out there are for he most part Home builds and made by limited production companies. Now Some of these cannons and morters makers make better products then Smith and Wesson & Colt others can be a bit questionable.. Also there is the operator error that can mess up the best BP cannon making it into a bomb..
So I can understand a range taking the Safe approach by not allowing them so they do not have to judge the safe cannons from the Unsafe, The Idiots from the Safe Cannoneers.
But I do understand your aggravation, I have a couple of cannons and hate that look of "are-you-crazy" when you want to bring it somewhere to shoot..