I always thought the real reason was was a financial consideration. The range operators use the spent brass to reload and re-sell the ammo. If you allow cheaper steel cases you lose alot of money on free brass. I always retrieve my own brass, but it sometimes gets mixed with other brass on the floor.
They're talking about ammunition with steel-jacketed projectiles, not steel cases (though I've never seen steel-cased ammo that wasn't also steel-jacketed).
I've shot CCI Blazer, which is aluminum-cased, at Tulsa Firearms many times and they just tell you to throw the cases in the trash when you're done.