In other words, ISPs hate it because it forces them to actually do their jobs and be ISPs. The Internet itself is "a zoo of endpoints on a shared medium", and ISP stands for Internet Service Provider.
e.g., AT&T could provide perfect service to the home endpoint, but the customer bought some aftermarket router from their cousin, who had configured it for Verizon. Customer calls AT&T to holler. Tier 1 support doesn't know what that particular router config GUI even looks like, so it gets bumped to T2 or T3. Ultimately to find out that the customer's cousin had hardcoded DNS to some internal Verizon system that's not visible to AT&T.
Repeat x100K. ISPs job isn't just "provide the Internet," it's also "provide all the troubleshooting for every non-technical customer who just wants to watch Netflix but doesn't even know what a router is"
No ISP that I'm aware of will provide troubleshooting for devices they don't own. They just say "sorry, not our device, not our problem". When I installed my own cable modem and router, Comcast was quite clear about that. And I said "fine, no problem".
Most people are unwilling to pay, and yell at customer service. Most of the times, specially when the router has sync it's customer fault.
People who don't even know what a router is don't buy their own equipment. Those who buy better routers don't require support for them, they call when there's a problem between the ISP and the router.
I used to install satellite TV and saw this all the time. People would get old receivers from friends and family. Fortunately for me, the receivers were proprietary to that service (we don't use generic pay TV receivers in Canada) and the old ones were built like tanks. If people could have supplied any old cheap generic receiver, I probably would have had a bad time.
No, the shared medium in this case is referring the last part of the cable network where everyone in a neighborhood is transmitting effectively onto the same cable.
All it takes is a single device with a broken configuration to spew crap onto the wrong channels, taking down the whole neighborhood. https://en.wikipedia.org/wiki/DOCSIS#Physical_layer
On the Internet it stops being a shared medium the minute it gets out of the cable network into fiber/ethernet switched+routed interconnections.
Is it really that easy for a single device to take down a neighborhood? For example, could a bad actor trivially disrupt a node with a modified modem? I guess that would be difficult to defend against.
I don't know enough about docsis to say if it has any protections against out-of-spec devices.
I don't know why I'm surprised when things outside my area of expertise are fragile, considering things inside my area of expertise are fragile.
Yes, similarly to how you could jam a radio frequency. Fortunately, this is rare and only ever happens due to hardware failures in modems.
If you want an ISP whose competitive advantage is dealing with whatever crazy shit the edge throws at it, then that is your prerogative. But having some ground rules and baseline behavior makes it so that the ISP can focus on more rewarding tasks, such as negotiating peerings, establishing direct tunnels, improving network observability, and predicting necessary backbone upgrades.
Doesn't "if you choose to use your own cable modem/router, it must meet the DOCSIS 3 specification" do this? That's the rule Comcast made me follow.
There is always a provider-managed CPE device that functions as the service demarcation point. This is the point where your contracted service speed is enforced (shape + egress queue and ingress policing).
You can have literally whatever router (dumb, smart, next-gen, whatever) spewing bits at X rate. The CPE will essentially normalize (police) that bit rate to your contracted speed (upstream scenario).
