Cox Communications Used Modems Useless by Policy Omaha, Nebraska

I purchased a cable modem off of Craigslist to save $80 on the retail cost of the Motorola SBG6580. I called Cox and had my service activated with this modem, only to have my service "Temporarily Disabled" a few days later, with a Cox splash screen indicating that I need to call Cox to resolve the issue. 

Technical support finds nothing wrong with my billing or service (besides a weak signal, which Cox always seems to have) and they have me resetting my modem, and checking for service only to discover after a couple tries that my modem is still associated with the prior owner's account. They tell me they can do nothing about this association, and that the release must come from the prior owner. I wonder how the FTC can let Cox put a "policy" in place that turns a piece of hardware you legitimately bought from another owner into a paperweight. 

The old owner may call Cox to release it, but I already paid him, and he has been unresponsive to my attempts to contact him so far. Seems a little ridiculous that modems are so tightly tied to accounts as to be almost "non-transferable" especially when they remain tied to accounts that have been closed.