Why should it be US only; other people speak US-English and want US English version of Cortana as well even if they are outside the US.
I want access to Cortana outise the US for English speakers. I don't care about language localisation, but for people who are English speakers or just US people living abroad, why should they not have access to Cortana right away with the rest of the US.
It should be up to the user to decide if the US localised Cortana is sufficient not on Microsoft limiting it's access.
Also how is it going to be determined who has access? Is it going to be based on the Phone ROM or on MS account location or on where you bought you phone with which operator?