I wonder how it'd work at regular retail stores. As it stands now, you don't buy a Steam Card and take it home. You take that card up to the register and they scan the UPC code that is on the back. The card itself only really has a UPC code on it and holds no inherent value sitting on the shelf. Also, the cards themselves don't get "activated". So like, if you wanted to try to leave the store with one without paying for anything, then you'd be the proud owner of a shiny, newish piece of cardboard. Anyway, after the person scans it a receipt prints out with a code on it. You take that code and enter it into: https://store.steampowered.com/account/redeemwalletcode
and then you have money in your Steam Wallet. But all you leave the store with is a code printed on a receipt.
This is something that is completely handled within GameStop's network infrastructure. If these were to appear at Walmart or Best Buy or something similar, their own computers would have to be upgraded to print out those codes.
On a side note, I bought two $20 Steam Cards. I activated one and gave the code for the other to a friend in Europe. My code activated at $20. His code activated at 15,47€. So they're usable everywhere, but at local currency exchange rates for either $20 or $50.