The thing is the signature on the back of the card isn't for verification by a merchant. The stated purpose of the signature block is that you agree to the terms that come with the credit card. By the rules of Visa and MasterCard a merchant should not accept a card that is blank or has something like see ID.
Of course almost no merchant follows this part of their agreement.
It's amazing to me how many people don't realize this, and think it is somehow safer to leave the card unsigned.
Back when I worked as a cashier (at Target, of all places), I actually had people get offended when I would ask to see their ID because their credit card was unsigned. But I know many/most of my co-workers didn't check...
Also in the category of things the store should check but rarely does -- The merchant is supposed to call the credit card's issuing bank before letting someone else use the card -- this also angered people when I would tell them I had to call the bank to let them use their spouse's card.