Generally speaking, employees in the US can be fired for almost any reason, as long as it isn’t for an illegal reason. While Amazon isn’t calling this “firing”, forcing someone to resign is essentially the same thing as far as the law is concerned.

I do not know how they can do this also, but it depends upon the State.

I think "blue" states Amazon would not be able to do this, "red" states I think anything goes. Red State Gov. only cares about not giving poor people money.