I can reiterate what has been said re the Canadian way.
A service entry section of a panel used as service entrance can have only the service conductors installed. No Branch wiring at all. There is also a requirement here for the main breaker section to have its own barrier as the conductors are still hot even with the switch off. The panel could have been installed with the main section at the bottom. I had a guy from Arizona use a 2" PVC connector for all the branch wiring and I rejected it as there needs to be a connector terminating each cable. That method is going to permit rodents getting in the panel, I also don't like using staples as strain relief since staples are not supposed to be driven hard enough to grip the wire so the branch wires could be pulled out of the panel or tight against the terminals. It does appear to me that this type if cable entry is a lot more common than I believed.
