Signature on the back is all that's legally required, it's not hard to get a fake ID, but it's very hard to fake a persons signature. Any store that requires me to show my ID and ends up not letting me purchase, I call Visa directly and report them. I would much rather have them match up my signature then ask for an ID. I've worked retail and if a store gets 2-3 of these complaints from customers who were forced to show ID, Visa will get involved, as they should. I'm going to go out on a limb and say Visa knows much more about how to prevent fraud than a person working behind a register. They require a signature on the back over showing ID for a reason.
You think they actually check the signature?
LOL