Activating those visa cards isn't as easy as you think. It is much like opening a checking account, over the phone. You have to give out a lot of information, like SS#, address, phone number, etc. I'm not sure someone under 18 could successfully activate one - but I could be wrong. As a walmart cashier, I also bet that you have to be 18 just to purchase one. (though that is one of the few items I have not yet sold). Cashiers are prompted to check IDs for so many items other than just tobacco, alcohol, and firearms, including: ammunition, inhalants, cold medications, and mature rated CDs, DVDs, and video games. If it isn't legal for an underage person to purchase one, I guarantee the cashier is prompted to check the ID - whether or not they actually do is another story.
I have actually activated one of those things, when my mother and law sent us some money on one of them, rather than a regular walmart gift card. Took about 25 minutes on the phone, and the phone system actually pulled up a lot of information on its own, in addition to what I was required to supply in order to get it activated, and there were pauses while the information, such as my SS# was verified. Like I said, any information you'd be required to give to open a checking account is required to activate one of those cards.