Cable modems actually never allow you to change the MAC address. You're thinking of consumer routers which commonly allowed cloning a MAC because old internet s

Cable modems actually never allow you to change the MAC address. You're thinking of consumer routers which commonly allowed cloning a MAC because old internet services would hardcode a single MAC address per account in order to prevent customers from just grabbing 10 public IP addresses each. Just about every single DOCSIS network still in existence uses BPI+ to encrypt the traffic to each customer to make sure that you can't just sniff traffic for the entire neighborhood and steal service. BPI+ has a certificate that is minted and signed at the factory with the device's MAC address in it. If you tried to change the MAC address, even if you had root, you wouldn't have a valid certificate so the modem would never get a connection.