It’s not for the activation or anything of that sort, but Apple requires you to use a marked form of payment for the transaction to help eliminate scalpers, a problem it’s had between itself and AT&T.
According to Apple it does this to make sure that whoever is purchasing the iPhone is the person who’s activating it, so that they can be informed of the policies, notably that not signing up for an AT&T iPhone plan voids the warranty.
A guy in upstate New York found this out the hard way when he went to buy an iPhone with cash and was told that’s not how it works. He was embarrassed, as he doesn’t have a credit card. Sure it’s odd, but many people don’t have a credit card for fear of identity theft and other privacy issues.