I expect they were trying to restrict muzzle-loading shotguns for some reason. I really can't imagine what kind of safety-related issue anyone might have strictly concerning black powder v. smokeless.
A black powder ban on an outdoor range would be close but not the most ridiculous range rule I've encountered. An indoor range in Vacaville, CA offered special fees on particular days each month. For example, on their "Glock Day", you got some kind of discount if you were shooting a Glock. They also offered a "Colt SAA Day". Oddly enough, Colt SAA revolvers and their clones were not allowed to be used in their range. The only single-action revolvers allowed required a transfer bar.
Never did figure that one out...nor did I ever do business with them again.