They leave the actual cables in place, in case you change your mind and want to go back, or you move out and the new occupants do, so they don't have to do any physical work.
If you're lucky, they'll ask for their box back, but in the three separate occasions I've subscribed to NTL and Virgin, they've never asked me for a set-top box or cable modem back
Which I presume is done on the same basis, in case you change your mind or a new person moves in its probably cheaper to take the odd hit on a lost box than send out a load of new engineers, risk losing people to sky. Certainly many of the student flats up here have a virgin box in "freeview mode" or a clapped out old pace box on a M subscription that was "there when I moved in".
I am sick of been persicuted by you immature TV Forumers!