It doesn't, at least not to me. I suppose there might be some who felt uncomfortable copying an "in print" cassete to CD who might feel more at ease knowing a new CD copy was not available but I see nothing morally wrong in the first example.

For me, the only reasons I can see for wanting to purchace a CD are quality of recording (?although I would guess it might be possible to make a CD that sounds better than the cassette? I know there are programs that try to remove unwanted noises from vinyl) and the time needed to produce the CD.