Are you forgetting the difference between P code and the C/A code (which use different frequencies)? The latter is freely available via the open market, which means anyone can build it into weapons - hence the possible need to jam. A low power transmitter feeding false data could totally corrupt systems using the C/A code. Of course you could just transmit noise, but since the GPS reciever extracts the signal from noise it may be hard to fool if the signal doesn't look right (it knows what the GPS signal looks like, and correlates this against the noise to extract the signal buried within it - so jamming it with white noise won't achieve much).