Many devices have internal hubs as well. My notebook chains one external USB interface through two nested internal hubs (the deepest of which also controls the mouse and keyboard) before it even gets to the physical port, the other USB port goes through yet another hub. There's three total hubs in the system before even plugging in anything external. A rule like this would kill all of my existing HID devices except for my webcam, which is the only device on directly connected to the USB host.