When the US and Canada penned the pre-clearance agreement, I believe that there was a provision that allowed CBSA to set up pre-clearance facilities at select US airports. While I am not sure what airports were involved, I remember DFW and LGA were on the list, but not SEA. I believe that in order for CBSA to begin pre-clearance, the agreement required approval from either Congress or through an executive branch rulemaking procedure.

~FAI PDX Flyer