Your absolutely right. And every single southern state had Soldiers fight for the Union, not so the northern states.
Traitors who would turn on their own people simply want to be on the side they perceive will win because of the greater numbers they had.
And I think you are likely mistaken about no people from Northern states fighting for the Confederates. If you count the border states as "Northern", then there were massive amounts of them fighting for the Confederacy.
Even not counting the border states, i'm pretty sure there were some people from Northern states fighting on the side of the Confederates. No, I don't have any examples, but the probabilities would dictate that it was so.