They waive that in a lot of cases, as they're trying to build out their network of xfinitywifi hotspots you see everywhere. I still ran my own modem, but it's a point to keep in mind.
They give you a combo router and modem by default when you run their modems (for the reason given above). The WAN port is just a downstream port for the internal router.
I don't think there's a way to disable the router itself in most cases these days, just some of the services it provides.
