Full pre-clearance when flying doesn't always mean full clearance. It doesn't take much for a pre-cleared plane from Canada to divert to the international terminal at the destination US airport for any irregularity. Doesn't happen often, but as a 2MM flyer I've experienced this a few times over the years - usually at JFK or MIA. You just hope your tiny jet has not arrived at the same time as a few wide-bodies from overseas. Also, some other countries with pre-clearance agreements - Ireland and Aruba come to mind - include only Immigration inspection, with Customs completed at the destination US airport.