Because there isn't any conclusive authority on the question. And it is a reasonable view as demonstrated by the attacks on Goldwater and Romney Sr. and the current press and McCain's concern in the Senate that it was intended to mean in the territory of the 50 states.
But it is an arguable question. I don't view the 1790 statute as having much bearing because it was passed long before we had possessions and territories outside the US and when we got them, we then significantly modified the statute to revise the rules.
Everything here is definitely arguable, but the 1790 law was absolute -- if you have two American parents then you are a natural-born American citizen, no matter where you were born.