The United States has what is called a world-wide tax system. This has two implications. First, as a U S Citizen or resident, you are subject to tax on all your income from whatever source. The second implicationis that income "effectively connected" to U S sources is subject to U S taxation. However, you are not a US citizen or resident and you are not performing the work in the United States. It is not considered effectively connected, therefore, it is not subject to U S taxation.