I charge $10-$15 (depending on size, how many passes to get em flat and weather there is greasy hubs to deal with or not) each to turn them and most have some extra material to do so, a shop not wanting to do it is because they are lazy/greedy... there is a bigger profit margin on selling new ones, they mark it up at least $15 each anyhow, they don't pay the tech any labor and the job gets done faster.

On the other hand I prefer to machine them for several reasons
1. The chinese material is junk on new rotors and they frequently come back warped where the originals or even used chinese ones have been heat cycled a ton o times and are not gonna warp quickly under most circumstances.
2. I live 2 hours from a real parts store (NAPA directly accross the street from my shop is a joke, they have to order everything, even have to order me ATF+4 so I can get the job done faster by doing it myself.
3. It saves the customer money and they love me for it when I save em $50-$100 even on some brake jobs.