Tableview:
As noted, given that the airline does permit cancellations, in order for the airline to prevail in court it would need very clear evidence that you never intended to fly the return. This would be very expensive for them to amass as to an individual flyer (granted if an agency or corporate travel department did this, it might be easier for it to assemble adequately convincing statistical evidence).
But further, if you, as an individual flyer, canceled returns on a particular airline frequently, this also means that you do fly with that airline frequently (something airline robots definitely do track religiously). Thus, I would suppose the airline might be reticent to try to sue a passenger from which it already collects a lot of revenue just to have a go at trying to collect even more. My guess is that: (a) they would have a hard time prevailing in court; (b) the public relations effects would be awful; and (c) they can kiss off ever getting any more business from that passenger.
Therefore, if I was HT, I probably wouldn't lose too much sleep worrying about an airline solicitor knocking at my door.
