I think the answer is that shops do not typically pay for the r-12 they recycle. There is some upfront capital expense on the part of the shop in purchasing reclaimation equipment.
Now that r-12 is like gold, an honorable technician might theoretically 'credit' you for the r-12 already in your system.
In reality, it is more common practice to charge you for the entire refill capacity.
r-134 conversions are a great deal for a shop. They get to keep your r-12, and charge you for the conversion! I would imagine that this is the primary source for 'onsite recycled' r-12.
1986 300E 5-Speed 240k mi.