I thought the civilian US population originally funded GPS via their income taxes?
Yup...but the US population paid it as an instrument of war and the USAF is maintaining it still to be used as an instrument of war. There is not guarantee of being operating at any given time. If the USAF decides to pull the plug...that' it...if you want to charge for the use of it...you should also lay down T&C and be liable when is out of service.