This is not correct; it's a matter of physical location. If someone paid by the forum was not physically present in the US at any point, then his income was not US source income, and therefore no US tax would be due. (Local-country income taxes probably still apply.) US tax could in some cases be due if the person spent any time within the US, but even then tax is not necessarily due, and it is not necessarily required to file a US tax return. See Pub. 519, especially Ch. 2.

(This post is not to be interpreted as tax advice.)