Don't forget that the GPS system's original use was primarily US military, and it has two separate usage modes - military and civilian. It wasn't that long ago that the civilian mode was finally switched to the same accuracy as the military mode. Previously it had been deliberately degraded in accuracy.
Anyway, it's also very likely that the GPS system can be deliberately drifted on command. If many CNC machines really do use their GPS position to detect when they are transported, and go into a locked mode if they are moved, that also means the machine's owners are effectively hostages of the US mil/gov. Any time the US decided it was time to shut down _all_ the high end CNC machines not under their direct control, they could.
Incidentally, if I was designing a 'movement tripwire' into a CNC machine, I'd use both motion detectors and GPS. With the GPS doing nothing if there was no signal (inside a factory) but averaging position over time when it could get a fix. Like when the machine was outdoors on a truck.
Next question - if high-end CNC machines have GPS built in, do they also have GSM data links installed? Car manufacturers do this, ostensibly to allow monitoring of the vehicle's mechanics for service and maintenance reasons. Why not do the same on a CNC machine worth far more than a car? Never mind that it's a huge security backdoor, right?
It's amusing to see this thread here, and Geoffs posting in it happily, since when I posted some links about the US government mandating GPS always-on position reporting via GSM in new cars, he deleted it. When asked why, he claimed it was some kind of conspiracy theory. Despite that it's nothing of the kind, and most new cars already have this. The gov now wants to make it mandatory for _all_ new cars.
Cars, CNC machines, what's the difference? I'd have thought the main difference is people's lives depend on cars working right, which makes the GPS/GSM backdoor/hacking issue rather more critical in cars.