You seem to interpret the current understanding of laws differently than the supremes. If, even, when both parents are citizens of another country but the child is born in the US, the child has, first and foremost US citizenship. Secondarily, he may take the citizenship of either parent, if I understand the current law.
At least there are thousands of Hispanic heritage ankle biters, commonly called ANCHOR babies, snot nosing their way around this country doing just that.
As to dual citizenship, yes, the supremes haven't had that issue served up as yet.