One of the ranges I used to use, charged by the hour. I didn't make sense to pay them for loading time, so I usually loaded a bunch of mags before heading out. You can go through a lot of rounds really quick with a carbine, and it's no fun standing at the line, or in the cold, loading mags.
Then there's the SHTF scenario, when you might need to bug out fast - its a heck of a lot faster to throw 30 mags in a bag.