Easy: all they have to do is specify excluding the counting of non-citizens only for certain states and count non-citizens for certain other states.
They can also specify excluding non-citizens of certain countries (Americas ex-Canada; Middle East; most of Africa) while counting non-citizens from other countries (Canada, Europe, Russia/satellites, South Africa).
What, you don't think they can do that? Who's going to stop them? You? You, Lieutenant Weinberg?
When it comes to breaking the Constitution, might as well be in for a penny, in for the whole pound.