I think that shoe removal is required in preclearance areas because the passenger will end up in the secure area of a US airport upon arrival. A flight arriving from an airport such as YLW will require customs clearance when it arrives. If the passenger needs to connect, he/she will have to go through TSA, which has the mandatory shoe carnival.
EDIT: here's the link
http://www.catsa.gc.ca/english/event...ice_avis.shtml
Passengers boarding flights to the United States from US pre-clearance airport must remove their footwear for additional screening. All passengers boarding any flights may also be requested to remove footwear for additional screening.