These are somewhat generic boxes by Motorola that Vz then programs to their own specs. Other companies may choose to enable the USB port, for example. (Or not).
Got me ... you'd have to ask Motorola. It's there for "future options".
The USB ports are standard on Motorola / Scientific Atlanta boxes and it would probably be more expensive for a cable operator to try and get a box w/o them (chip set, chasis, etc.)
AFAIK, they've never been enabled for any user-visible features (by any cable operator). They are powered though ... I've got my blackberry charger plugged in to one.
Dunno about the Motorola boxes ... but with the Scientific Atlanta boxes the USB port can be used during the development cycle. Software developers can use the USB port to load software onto the box for testing and debugging w/o the full back end / cable plant set up.
There was also a company playing around with diagnostics via USB (for field techs). Field teck plugs their hadheld into the STB USB port ... gets some handy-dandy diagnostic info. Dunno if that ever went anywhere / got deployed anywhere.
Oh yah ... and some of the Scientific Atlanta firmware versions come with a simplistic USB keyboard driver. You can plug a USB keyboard in and it will recognize certain keystrokes (numbers for channel changing ... + / - for channel up / down ... etc).