That's mostly a decision to increase turnaround speed on their bridges, it is faster to swap a rotor for a new one than it is to turn one down. But if you're cost conscious enough to replace your own brake pads and you don't want your new shiny pads to be just as bad as the old ones after a couple of weeks (as in: grooved, they will likely still be thicker) then turning the discs is a good option. Where I live this is still pretty easy, it might be harder in other places to find shops that will do this. Cost me about 10,- / disc.