I am not sure what they do with it, because you could recycle it in to other petroleum products, or you could burn it for fuel. Up north I beleive its pretty common to run a used oil furnace.
Every place I have worked locally, we sell the used oil to a recycler, and use the proceeds to pay the dude to come get the used antifreeze for recycling.
I have never seen them turn down oil, but let there be just a little contamination in the coolant and they don't want it.
Disposing of waste gasoline is incredibly expensive. $11.00 per gallon if I remember correctly.
Our oil, brake fluid, tranny fluid, axle oils, and whatever assorted nasty chemicals we use with the exception of antifreeze all goes in the oil buckets. We are almost exclusively using synthetic lubricants, they are handled the same way as regular oil. Down the same bucket!