USB sockets are often fitted to devices like STBs so that firmware can be loaded at the factory or/and repairs/updates in the field. Even if you could activate it, the box would need firmware to support any function, for example an external hard drive, that could use it - which it won't have.
Some USB ports are on devices just because that is how they come from the OEM supplier and are never used at all.
In other words, just having the port doesn't mean that is is of any use.
T, January 2011