Some of Apple's chargers use non-standard mains connectors based on the IEC320 C8 inlet and a stud (?). Said stud is plastic on double-insulated units and metal on earthed ones, and that's the only difference.
The problem this poses, of course, is that it makes it possible to unearth one of those supplies with no modification of any connectors or wiring. (e.g. If you lose the original Apple cord, the only standard one that fits is the IEC320 C7, which, as everyone knows, has no earth pin.)
At the very least, whoever made this decision should be fired from Apple.