[yummyfajitas]

...Saverin benefited immensely from being in the US (and that the stable government supported by taxes absolutely played a role in his success).

This is irrelevant to the question of Saverin paying individual income taxes. Any foreign investor gains these benefits and pays for them via corporate income taxes and the like.

Should all foreigners who make investments in the US be subject to US income taxes? If not, then this argument is moot.

[geebee]

No, I think that income earned in the United States should be subject to US income taxes.

I would consider stock that was earned while working in the US to be income earned in the US, even if they don't become valuable and aren't exercised until a later date.