if some mad group of god knows who, decided to wage some GPS enabled mayhem on US soil
If that should ever happen, heaven forbid, it would place the US gov in a real buggers muddle - the generousity of free GPS flying straight in the face of national security. Indeed too many countries, incl US, have placed all their huevos in the GPS basket, infrastructures that relay on GPS derived time & position. So many industries out there are constantly looking for new applications and business opportiunities based on this free-for-all asset until someone pulls the plug.
Remember the Y2K concerns, GPS calender roleover etc ? Such is the worlds dependence on software and GPS time.
On this basis it is easy to undersatnd other country's reticence to placing critical aviation requirements on GPS only.