They have to keep sending out an analog signal for all those entitled basic access. That is a wire coming into the home so as to use their line to receive the basic over the air stations plus all the public access and govt run stations and a handful of others who basically give their service for free (USA, TBS, CNN... I think those were on the basic service when I had TWC (NYC)... not sure if it's changed and the line up does vary depending on carrier.
It's under the same provision or FCC rule as the supply of a telephone line. I think it's a legal right to lines of communication or some shit. I think it also covers the entitlement to mount a DSS dish too.