you exited the international flight, went through customs, picked up your luggage, and then if you were continuing on to a domestic flight (which required getting past the secured area to the monorail), then you had to go through security again.
at this point, you could no longer have anything in your checked baggage that could not go through standard TSA security, as you were already inside the secured area. some airlines (Alaska, for instance) allowed you to check your baggage PAST that security area, but you still had to make it past.
since you had already picked up your checked baggage, and no longer could get to any connecting flights without either exiting the airport completely (far away from other flights), or going through security again with your luggage to continue to the monorail, you were pretty much screwed.
editing to add: there were a lot of very unhappy people who had their airline provided or purchased past security water bottles being taken away before they could continue to their domestic flights. I have no clue exactly how far I would have had to walk outside of security to get to the domestic terminal, but my flight was so soon that I would never have chanced it.
double editing to add that I've flown international through other airports and never had that type of experience other than through Seattle.
triple editing to note that even SEATAC seemed to think this was a mess and the new international arrivals terminal is due to open "soon".