Outside of the 24 hour window there is no reason to allow a voluntary cancellation for free, unless the fare fules applying to that ticket allow for cancellations at any time without penalty.

Given that you are asking here, rather than ringing to cancel your ticket, it sounds like you have not (yet) found yourself in the position of wanting to cancel beyond the 24 hour window. My advice? Don't book a ticket until you know you want to buy it. And don't come up with any strategy which involves needing to cancel for free a ticket outside of the minimal cancellation window. Use the "take an option" instead - for a nominal payment, which is removed from the total purchase price if you go ahead and book the ticket within the allotted time, you can "press pause" on a particular ticket, and go through with the payment at a later (but limited) time for the same price. If you don't purchase the ticket, you lose the fee - which is better than losing the entire purchase price.