It may be like cellular phones: even two of the same model phone (let's say Nokia 3560's) can have different frequency band chips (where the ESN resides, I think) and/or standards. A 3560 that works with Cingular may not work with Suncom, etc. It's a firmware thing.

Unlike the phone system (where each line has a wire pair back to the central office and any phone will work on the line it's plugged into), cable TV is sent on a single cable and split a few thousand times. It's the addressability that enables the cable co. to activate a box.

When the cable co. injects a command to activate/deactivate a box, all boxes on line receive the signal (like X-10 modules), but only the specified serial number (house/unit code) responds and operates. There used to be filters you could buy that blocked this signal.

The cable co. may be forced to allow you to use your own 'compatible' box, but they're apparently not obligated to provide you with one, or even make sure you can actually find one. Maybe there's a cable co./box-maker collusion class-action lawsuit brewing here.