[superuser2]

The cable modem is telco infrastructure that happens to be in your house. The boundary between telco and customer networks (called the "demarc") is between the cable modem and your router. It's entirely theirs to administrator, same as the vault down the street.

A comment I posted about this a few years ago: https://news.ycombinator.com/item?id=6998650

[jwn]

It's not quite that simple. In some cases it's telco infrastructure that's owned by the customer. In that case I'd say the demarc is like to be somewhere inside the modem, which is sort of nonsensical.

[superuser2]

When you own the modem, you're free to disconnect it from the cable company's network and do something else with it, but administrative access is still a condition of service from your ISP as long as you continue to contract with them for internet access on it.