Q: can't gift app without credit card

You can't use store credit to gift items, you can only do so via a credit card : http://support.apple.com/kb/HT2736

If you want to put more thought into giving an iTunes Store gift than sending a Gift Certificate or iTunes Gift Card, give a specific iTunes Gift. This allows you to personally pick a song (or create a playlist of songs), album, video, audiobook, or app to send to someone. You will need a credit card to make the purchase.

Because it says you can't? That would be my guess. We don't make the rules, we are users just like you.

At the moment Apple don't allow it, you will need to use a credit card - whether they've ever said why I don't know, but possibly it's to try and make it more accountable.