Car analogies are always fun, so here's one: The catalytic converter on my car is sawzalled off in the middle of the night. I drive around with the missing cat for a couple months. Then I fail a smog check, or get a ticket for loud exhaust. Is it my fault the cat was stolen? No, but I still have a responsibility to everyone else to not pollute (fumes or noise).
If the ISP supplies the router, then of course they should correct it. But if the subscriber brings their own equipment, then they take on the responsibility of maintaining it. Including replacement if it turns out not to be fit for purpose.