Hi,
"No steel bullets" has been the range rule at every SoCal range I've visited for a number of years. Simple reason: fire danger from bouncing steel off rocks, and most of the ranges are in rocky foothill areas prone to wildfires. My current club has had one small fire that I know of started by some of those sparks, and requires members to have a full sized shovel in their vehicle when on range property, "just in case."
At that club, we're kind of on the honor system, combined with some common sense. It's been raining for a week, and the hills are green and wet. So the range officers may allow a little leniency. But in a couple of months, when everything's starting to dry out, they'll get hardnosed until this time next year. Other ranges I've visited will check your ammo, and anything magnetic will earn you a trip home to leave it--can't even promise to put it in the trunk and not use it!
My only thought about disallowing the steel cases is that until recently, one was most likely to find them in Com-bloc milsurp ammo, which almost always also has a steel core bullet of some sort. CYA?
Rick C