Well, I just spoke to one of the officials of the transporting airline on arrival and it appears that this may not actually be a CATSA policy but rather a request of the airline. They have been having a fair number of passengers who have been using fake onward boarding passes to access certain areas of the terminal during transit stops and then claim asylum in the UK. This is one of the steps the airline has requested be implemented. The person summoned (who I presumed was a CATSA supervisor) was actually the airline security manager and he had even sent a message ahead to watch out for me on arrival in case I tried to claim asylum!!!

I guess I gave poor CATSA a bum rap here for no reason. My apologies for what appeared to be a rather assinine policy on first glance but makes sense now.