But "use contactless" isn't what that option does. It cancels your transaction which is why it's labelled cancel.

Suppose I'm at this screen about to get myself a paper ticket to Brixton to see my friend Jim, as this prompt appears I see Jim - oh that's right, Jim is coming here we're not meeting in Brixton. Cancel. I'm not making a journey, I don't want to "use contactless" I want to cancel this purchase, and that's exactly what this option does.

Yes most users who choose to cancel might end up using contactless, but that's not what the choice itself does, it does not, for example, check that you're carrying some form of contactless payment, nor does it charge you for a journey, it just cancels the ticket purchase.