One of the founding principles in the Declaration of Independence is that no State has the right to exist without the consent of all people 'governed' within that States area of control.
Any American stating that Israel has a right to exist or demanding that others state that Israel has a right to exist are unAmerican. And being unAmerican in defense of a foreign State.
Fucking traitorous scum.
So what?
The use of the question: Do you believe Israel has a right to exist?
is a propaganda point universally used as a gotcha against critics of Israel.
Furthermore, while the view that States have no right to exist without the full consent of all of the governed is a (hypocritically held) American view, it is a universal one held by anyone who believes apartheid is wrong.