So reading this, the US takes the position if a non-US citizen was issued a green card and they no longer live in the US, they are still subject to US income taxes on any worldwide income they earn.
I find that hard to believe as it's equivalent to financial slavery over subjects not only who are not US citizens, but not even residing in the country. Then again, the US is a hegemony and they do think they are the world's police but fortunately those days I believe are about to come to an end. At least, let's all hope so.