My range is indoors and in a metroplitan area of Southern California. Most of the ranges around here post signs that say " No holstered weapons allowed." At my range the sign reads "No holstered weapons unless you are LE or CCW". So they don't even want a holstered weapon on the premises. On the range, for open practice (other than a closed class for example) no drawing from the holster is allowed, CCW or not. When the LE come in (mainly Border Patrol and Sheriff) they close off one of the two rooms for those guys so they can draw if they want.
At the big outdoor range in one of the canyons north of LA, same deal. Holstered firearms allowed for LE only and no one (including LE) is allowed to draw/shoot from the holster.
I'm guessing it's pretty standard everywhere, but then again I live in California and we're pretty squeezed when it comes to firearms.
I believe it's a safety thing, mishaps being more likely to occur coming in and out of a holster.