The same officials from the US do clearance and pre-clearance and have to work with the same rules. If you arrive in the US for clearance there can be very long lines if overseas flights have arrived recently. Prepare for long waits unless you have Global Entry (they don't seem to use NEXUS in US airports). YTZ has no preclearance and Porter flies to BOS and EWR where there are often long waits. Porter is trying to get pre-clearance.