The key there is that if you can physically connect to the device in question, you are rolling vs the device firewall, no longer against the host firewall. This is why the comment in the rules about not slaving things you can't physically protect. If you are wirelessly hacking a camera, it has the hosts defense. But if the mage makes the decker invisible, and the decker opens the device case to access a port, he has a direct connection, therefore bypasses the hosts defenses.
I could easily see a site having external video cameras not slaved to a host (or to a separate host), and having the internal cameras, doors, security, etc slaved to a different host, since they can reasonably prevent direct connections on the interior stuff.