Why is this so hard to understand? Router broadcasts a second SSID. Router ensures that connections to that SSID are completely isolated from your own network, never touch your IP address or internal devices, and are tunnelled back to base over a separate channel VPN.
In scenarios where you are maxing out your net connection and someone else is using the guest wifi too, your traffic always takes priority. That's a rule right there - the kind of thing that routers are kind of good at.
It also means that you have access to free wifi when you're out and about, as you can use your Comcast ID to use this same wireless in other locations.
As has been pointed out several times, many ISPs around the world do this. It's simple and convenient and not remotely worthy of the scaremongering and technical ignorance being displayed.