Already other manufactures have taken a slice of the pie by offering spare parts, rods, fittings and coils that are compatible with the Minelab detectors. Whist this continues to occur, they're likely to be losing sales on their own, identical products. Selling a single product (cameras, cars, detectors etc) around the world costs millions to design, research, manufacture, patent, register, market, promote, transport, distribute, display, insure, staff and sell. In order to sell the control boxes as an accessory, they'd have to lower the price to make it attractive. People don't like paying too much for something if they feel they can obtain it cheaper by other means or if it doesn't reflect their own ideals of value-for-money. Which is the reason this thread even exists. But X-thousand sales of a discounted unit means a massive profit loss... or just an enormous loss of gross income. And that's bad for business.
/Just sayin'