There must be a law that requires them to do it, right?
Well, it seems it is not the case [google video].
The Constitution specifically forbids any direct tax on wages and salaries, and the Supreme Courts consistently ruled in the same way.
Not a single dollar of income taxes is used, as widely believed, to offer and sustain public services.
It was an interesting video to watch, and btw I am not an US citizen, have no affiliation with any of the authors or organizations cited or whatever."