It's not
required, that's just how you provide a billing address so that when sites do an address verification, it doesn't fail. The page you linked says exactly that. Sites aren't blocking you from using a gift card, they're blocking you from paying without address verification.
> To make an online or phone purchase, you will need to register your card. You can do this by either calling the issuing provider, or by visiting their website. This information should have been supplied to you when you received the card. It may also be printed on the back of the card. When you make an online or phone purchase, the name, address, and phone number you use will need to be exactly the same as the information you provided when registering your card.
http://usa.visa.com/personal/cards/prepaid/gift_card_how.htm...