It's not the box that does this, but the cable company. It is a legal requirement for the cable TV companies to continue to provide the terrestrial five channels in analogue for the foreseeable future.
In fact, using a multi-standard TV, you can often get other free channels such as UK History and CNN this way too.
The Pace Twin Freeview PVR (personal video recorder) box allows can drive a second TV from it's second tuner. This is the only box that I know of that can do this at the moment.