When Ken said "Digital TV signals" he meant "over-the-air digital TV signals from an antenna."
The CECBs that are the subject of this forum are specifically aimed at converting over-the-air digital channels to analog for use with old analog-only TVs. The government is subsidizing these boxes because they're taking away the over-the-air analog channels.
The government is not taking away analog cable channels, so why should it subsidize cable converter boxes?
There are some boxes that pick up both over-the-air (ATSC) and cable (QAM) digital channels. But they're not subsidized by the government so you have to pay more for them. These boxes are discussed next door in the "HDTV Technical" forum. Note that these boxes pick up only channels that your cable company does not encrypt. Most cable companies encrypt all their digital channels except the local broadcast channels and a few odds and ends.