As far as I know, the security point has always been there - it had to be. I can't state this for certain but I *think* I've seen it there for the last 13 years. Never arrived in SEA on an international flight.

I think the reasoning for it being in the satellites is that the two carriers (NW in the S, presumably it was PA in the north) could clear pax and then transfer them to connecting flights. It isn't the best airport for pax terminatng in SEA.