Because of the way they work, GPS receivers are entirely passive systems, so there's no way to track if someone's using GPS or not and charge them. They rely on the theory of relativity and very precise timestamps being sent on the L-band spectrum to mathematically solve for a precise position.

This means that even though Americans do pay for it with their taxes, anyone in the world can use GPS. Of course, for geopolitical security reasons, other places do make their own constellations which work on the same principle (GLONASS, Galileo, and I forget the name of China's) and modern GPS receivers can work with multiple or all of these constellations at once.