For comparison: you can bring your own DOCSIS modem to a cable network, even though all the houses on the street are connected to the same cable and you could jam it, or send a voltage spike to break everyone's modem.
Not very familiar with DOCSIS and cable; the story I'm getting from my nearest friendly LLM is that while you could bring your cable modem, it'd have to be a pre-approved model, and that the firmware and configuration would be under ISP control, unlike with DSL modems. Is that wrong?
By law the demarcation is a passive one; the provider is not allowed to mandate you operate ANY of their active hardware. If they want to sell you internet only via e.g. RJ45 Ethernet they better consider asking your landlord to rent them space and power and Cat.5(+) wiring access to put a switch/router, because by law they can't dump that on you the residential apartment renting customer.
You may either rent/buy a device from your ISP, or you may bring your own, at your discretion. ISPs are required to accept all devices, of course if your device kills the network segment, they will kill your connectivity. But they can't refuse to let you connect.
You get taken to court and sentenced to pay the damages? Same thing that happens with the TV cable that runs through the whole street. Or the cars parked openly along the road. If you damage it, you pay for it.
Controlling your hardware without consent that they legally can't ask for would be illegal hacking.
They do however have the right to mandate certain configuration parameters just how they are allowed to mandate you connect something that isn't a noise generator to e.g. a cable TV outlet. Well, being able to limit you to connect devices that conform to some spec.
