I personally think this is intentional to keep people from plugging an extension cord directly to the block heater cord.
I'm in fairbanks, alaska. The typical winterization treatment here is that the dealership install a power strip between the engine bay and front fascia. They wire the block heater, battery heater, oil pan heater, and optionally the tranny pan heater to it.
A really common thing is that people back out without remembering the unplug first. I definitely wouldn't want to risk that when the block heater and extension cord are directly connected. If the plug doesn't happen to be oriented to disconnect, real damage can result.