1. in the old days users would load illegal modifications and steal bandwidth, while this does not stop it, it makes it harder.
2. Certain firmwares are required for CERTAIN cmts's which are the cable companies property and not yours, so it is best to let them update the firmware for you so they use the CORRECT one.
3. The modem manufacturer actually works with the cable mso to develop the firmware updates, and last I knew they do not release them to retail purchases anyway.
4. Whats to stop arris fromn killing your purchased modem? The Federal Trade Commission,
https://www.ftc.gov/, unless they'd like to stop being allowed to sell their product on US soil. Not to mention how would they prevent killing the MSO's modems they bought from them too, in which case they'd be shooting themselves in the foot becasue I guarantee you its a bigger market than selling them in Best Buy.