I have read that the Firewire is version 2, 800mb/s: if so, this has one huge advantage over all flavours of USB, in that it can operate with far greater cable lengths (100m compared to about 3m?). That could make it a great attraction for remote camera operation in sports arenas, big studios, and such. So maybe Canon views Firewire as the primary interface, with USB almost a token gesture to compatability.
Still the economic or technical reasons for saving a couple of dollars on the USB hardware is a mystery!