While all advice has been great. I 'd like to add from my experience managing a head end for a TV service (not cable) that although over compression of video is sadly all to common, it is not always just the fault of your local cable reseller.
As Alan pointed out, the networks themselves are free to manipulate their signal to whatever degree meets their quality verses costs needs. Add to that that the networks signal very often goes through a 'middleman' if you will, before it is sold again to the final provider. This reseller may also add compression in order to save on satellite uplink costs and other forms of bandwidth they might be using to resell the signal. And yes in the end, the local provider may add compression to the signal yet again to squeeze more channels into their available bandwidth.
Generally, they are all much more careful with HD programming but you can see where the domino effect can come into play.
