a great number of people with a very real interest in jamming the GPS system
Indeed, but the logical thing would be to use a very low power jammer, mounted in the actual vehicle.
Wholesale GPS jamming would stop GPS road navigation which many commercial vehicles rely on totally. It's also impractical; you would need a transmitter suspended under a baloon, or similar. Who is going to bother?
A 1 milliwatt jammer in your car would totally circumvent any road charging scheme but you would have to turn it off if you want to use a GPS for road nav yourself.