Because USA has made it a crime to exchange money without license or anonymously. In their eyes this is hardcore criminal behavior deserving of decades or even life in prison. USA's law applies in all countries in the world including any other planets such as Mars, Pluto etc.

The only way to protect yourself against the US is if you are in a country that is enemy of the US and will not extradite you, however they can still use drones to kill you.