“.why don’t they set up a RF DF “
That could work, but the difference between a “drone” and an RPV is that a drone is able to be programmed to perform a mission without operator input. Military drones tend to operate on some combination of autonomous and remote control. A mission involving going to a grid coordinate, and taking a bunch of pictures could easily be done autonomously. And drones don’t need to be just for governments any more the technology has gotten to a point where it’s not that hard to build a drone, some people do it just for fun.
I believe that the new models have built in GPS restrictions on height (under 500’) and sensitive areas like airports etc will prevent flying.
The makers of the popular Phantom series has the newer more capable Inspire Quadcopter with a built in stabilized camera system and dual control units, and the GPS restrictions.