Interesting question - especially so in cases where the taxes and other associated charges exceed the cost of the ticket.

Unfortunately, I am unable to answer your question with any certainty although if my experience is anything to go on then I'd say yes.

On one occasion I had booked a return flight with BA to travel from EDI to LHR - a non refundable/non changeable ticket. About an hour before the outbound departure, i phoned BA to let them know that I running too late to catch the flight. At the time, I thought a courtesy call necessary since I had checked-in online the previous evening - I was hoping they might put me on standby for the next flight but unfortunately BA were sticking to the rules on this occasion. My options were to buy a new return ticket (transfering taxes from unused ticket) or take the train. The first option was prohibitably expense so I opted for the latter. The BA agent proactively informed me that the taxes and charges (GBP28.40) would therefore be credited to my VISA account.

Whether the airline was required to do this I am not sure. It might have been a gesture of good will.