In a nutshell, you need to give C/C details because Apple probably want you to have a formal account. You may say you don't want one ... and there's an answer to that.
But assuming you are open to persuasion, the need for an account I imagine, is because neither Apple (nor you, for that matter) know exactly what you may want to buy or get free from the Mac App store.
An account allows them to keep their records straight and you to buy or receive free whatever you want ... with one click. No great punishment is it?
David, you'll probably want a paid app soon. Why not bite the bullet for the sake of future convenience.