OP, it's not necessarily sparks from the guns, it's from a still burning/sparking case landing on the ground. I can attest first hand to a case coming out of the gun, landing on the floor while still emitting sparks and igniting the unburnt powder on the firing line. This happened to me with a pistol round, IIRC it was the infamous AMERC. Or I may just be attributing it to the worst ammo on the planet. Case ejected, landed, I noticed the sparks in my peripheral vision, and in the time it took to look down at it, the point in front of the lane and the next couple of lanes went up like a flash fire. The range guys keep a 5 gallon bucket full of water for just such an event. At the time, the shooting area up to the firing line had carpet on it. This was at an indoor range, btw.
As to the steel cased ammo, at this range, it was more due to the fact that back then, many of the steel cased stuff had steel jackets as well. So, they just kept/keep all of it out. At another indoor range I go to, they don't care about steel or check your ammo.
As someone mentioned above, their house, their rules.