This isn't about the laws of electricity but about how the power supply is designed. I was surprised recently when my Mac-using colleague needed a three-pronged extension cord. I don't carry those around in my backpack because everything I need is two-pronged. I thought it was silly that Apple would force this on their users. With a regular transformer/diode bridge/capacitor power supply this isn't needed.

The traditional use of a ground-plug in these situations is to protected the user against electric shock should "hot" a wire get loose and touch the metal case. But when the voltage is dropped to non-lethal amounts (or if you have a plastic case) this is unnecessary. In fact, most "third wires" don't even make it to the device- they terminate in the transformer box. So the Mac plug was truly a bit perplexing.

But now I know why.