At the heart of the problem is that a GPS signal that does not use the coded method would need a lot of power to overcome the normal background interference that would occur over such a long distance. This would require significant transmitter power and hence electrical generating capacity on each satelite. It also minimises the interference with other electronic transmissions.
A more detailed discussion of signal to noise ratio can be found here
http://202.114.120.10/gnss/docs/AN101.pdf
Jamming and spoofing is possible of course but the transmitter power required to prevent this is truely phenominal and the electricity generation on board the satelite would need to be very impressive.