I do think cancel is a poor example because it's conceptually very near "go back" - but it doesn't actually mean that. It means "I've changed my mind".
Implementing cancel with a URL is an implementation detail. That's one way to do it, but it's far from the only way. Cancel could also be implemented by hiding a UI element, for example.
> Cancel could also be implemented by hiding a UI element, for example.
Sometimes this can be confusing, because the hiding of the UI element doesn't prevent the submission of the form later. If the page is a giant form, with individual parts made to look separate (such as a large tax form), the cancel merely hiding the edited element (while retaining the data) means a later submission could inadvertently submit the data that was intended to be cancelled by the user.
Implementing cancel with a URL is an implementation detail. That's one way to do it, but it's far from the only way. Cancel could also be implemented by hiding a UI element, for example.