Probably catering to the airsoft sniping competitions going on in Hong Kong,Nippon,Taiwan and such,like Mack said. These are definitely not made for skirmishing. But perfect for precision sniping competitions as you can remove any variable from bad ammo to get the best gun possible and thus only be worried about one variable- Your shooting skill.
Another option is to use 6mm ceramic ball bearings or even steel ball bearing but those are in the .80 or so region(But ceramics are about .6 something),so .66 is a good enough weight. The price is big but being CNC machined and reusable it sounds like a good option for someone seriously into precision sniping comps,being even more precise than using ceramic ball bearings.