How so? It's extremely short range, the only thing that can read it is the terminal you were about to give your card to anyway. It's not broadcasting while in your pocket, so nobody's going to read your card by bumping into you.

Magnetic field falls off at the rate proportional to one over distance cubed. A huge NMR magnet that will suck you in by you wrist watch at one meter is a harmless hunk of metal at four meters (approximately).