I was reading some old Kia recall not too long ago, and they mentioned there are only like 3 main companies in the US that do this service (why only that many, I'm not sure,) and that all the major car companies have to use them to have the digital mileage changed.
An obvious guess would be that there are only a few companies that manufacture the actual gauge cluster components, and those companies hold some sort of patent or whatever on the technology, so they don't allow the dealerships themselves to have access to it.. (think 20 guys who invented this technology knowing the secret to editing it, versus 30 thousand car dealership service dept ex-convicts having access to it.)