Well, they can "assess" the value of the ticket all they want, but other than filing suit I seed no way for them to collect it (unless you bought it through a travel agent, in which case I suppose they'll send a debit memo to the travel agent and make the poor agent pay the difference between your fare and full Y).
What if you buy a roundtrip ticket, take the first flight, then drop dead??? Are they going to "assess" the fare difference from your estate??
I think that as a practical matter the occasional thrown away ticket doesn't matter. If you established a pattern of it, however, they might take you to court and use the contract language as the basis of their case.