Is it any wonder that someone who goes to great lengths to hide the printed face of a credit card slip might be a lousy tipper? And just to be safe, lest an errant breeze should traverse the room and lift the folded flap of the paper, exposing their cheapness for all the world to see, a pen is invariably laid across the slip--an extra barrier for the server to remove before being met with the inevitable disappointment.
Perhaps, you say, the person is only trying to protect their personal information by folding the slip of paper. We, after all, live in the age of identity theft and cyber-criminals wearing special credit-card-slip-reading glasses are doubtlessly clinging to the very ceiling above with the use of suction-cupped gloves and knee-pads. It is easy to say that if you've never lifted the obstructing pen and peered between the folded leaves of paper, only to be met with a sum crafted a mathematician of the vilest, most black-hearted stripe.
In a perfect society, people would be forced to wear a digital display on their foreheads, indicating the lifetime tipping percentage.