I haven't heard of this happening, but according to this somewhat dated fact sheet from the Federal Communications Commission (FCC) , the cable companies are allowed to bill for the wire once the customer bails. Or they can show up and remove the home wiring altogether if the customer doesn't pay up.
This just struck me as interesting, considering that most new home networking installations from satellite companies and phone companies -- cable's worst enemies -- assume that homes will be wrapped in coax. (See RBOCs Want Inside Your House.) It seems that, if they wanted to, cable companies could really stick it to canceling customers and competitors if they started charging for wire.
Here's the juicy bit of the FCC cable wiring fact sheet:
After a subscriber voluntarily terminates cable service, the cable operator may (1) leave the home wiring in place, or (2) notify the consumer that it will remove the wiring unless the consumer purchases it from the cable operator on a per-foot, replacement cost basis...Cable companies lead the world in drumming up ridiculous charges for canceling service. But has anyone ever actually had to foot a bill for old wire? If so, do tell.
If the subscriber does not already own the wiring and declines to purchase it from the cable operator, the cable operator may remove the home wiring within 30 days of the subscriber's refusal, but it must do so at no charge to the subscriber, and it must pay the cost of any damage caused by removing the wire.
— Phil Harvey, Coax Facts Editor, Light Reading