Visa has leveraged its position to introduce a payment method that appears to the seller to be a credit card, but actually uses pre-paid funds. As a result, the user of the card does not require credit, and furthermore is not required to be of legal age. That's the target, in fact. Visa Buxx are targeted at teens. Teens are a potentially vast market for online commerce, not merely because they shop a lot.
Teenagers are also more likely to use the Internet, and to feel comfortable with online shopping.
Getting them early isn't a bad move on Visa's part, either. Of course, the disadvantage is that it could make it harder to monitor teens' expenses. Note the strong similarites to the Y-creds British product.