I do see your point about modifying the hardware when it's no longer connected to a public network but cable infrastructure is a shared medium. The actions of one customer can effect hundreds of other customers. For example, we saw a specific firmware revision that caused the modem's burst modulator (upstream signal) to turn on a high power constant carrier in certain circumstances which would DoS all the modems on the same upstream offline. I've heard this was responsible for a massive outage in Phoenix last year due to 8 modems being upgraded before the code was fully tested. If you upgrade the firmware yourself it's putting other people at risk -- including digital phone subscribers who may be using their services as a life line. Changing your MAC address has some risk simply because MACs are supposed to be unique. You could accidentally DoS another customer offline by doing it although statistically the chances are pretty low you'd use another subscriber's MAC but it does happen from time to time when someone simply increments the last digit of the MAC address.