Most ISPs use configurations which expose connected devices and their identifying information to the ISP. There was a good thread on it yesterday but I don't have it saved.
The major ISPs in the US, in my experience have always allowed either opting for a straight modem instead of one with router/switch/firewall, or at least let the customer set bridge mode so it's passthru.
If this is ever not the case, it ought to be a big story and controversy, because the subscriber then could not get the connection entirely under his own control (and two firewalls and/or double NAT can cause problems).
That was the main thread in the discussion yesterday -- you're renting access to the provider's network and the endpoint connection hardware is a part of that. Even if you buy your own modem, the configuration is controlled by the ISP through the setup and the router (again, even if it's yours) will also be configured to allow some backdoor information for the ISP. I may be misremembering a bit of that.
