It's also not true to say KFC flies exclusively Purolator and Morningstar flies exclusively Fedex. Every night, depending on space, you will see packages from most of the courier companies on a Purolator flight. There is also Puro freight being carried by Cargojet. Barry Lapointe is also a minority shareholder in Purolator, so he's also the customer and supplier.
Sorry DanJ...while you've got some of that correct, KF does fly for Purolator only. It's an exclusive contract. What you're seeing when you see CJ, Puro, or FedEx cans on competing aircraft is deals that have been cut by FedEx or Puro or CJ (UPS, DHL), not by the carriers contracted to fly their freight. All of the freight companies do it - it's a way of maximizing efficiencies if they have only a small amount of freight, or freight that might get bumped - and the other carrier has room. Lots of side deals. So while their may be mixed freight on board the aircraft on any given flight, it's the customer who has dictated that, not the carrier.
Pretty much what you said, but I think it's an important distinction as (at least in the case of KF and Morningstar) there is one customer that we're loyal to, and we're not out there flying the competitions freight without their direction.