How does being able to reprogram a GPS device make it into a jammer any more efficiently than grabbing three pieces of coal and running a few amps thru it? Or hell just buying an SDR on aliexpress!

The only reason it's "illegal" is because they were thinking people would use it to make missiles easily - but that's already the case even with non-reprogrammable gps. And in big 2025 you can also just use drones with bombs attached to it.