They don't charge extra. I wondered the same when I was asked (T-mobile for me though)
1) It's an option for people without credit cards (like younger ones) to be able to purchase things
2) Believe it or not, the carriers want you to need your phone and, by extension, them. Including purchases and using your phone like a credit card (like NFC tech) that way they get more revenue and things like that.
I remember that from a Nokia ad for the N9 talking about its NFC, how it can be used as a credit card, and how carriers want your phone to be a credit card-- their credit card. Though they didn't say it in so many words XD