I could never figure out why it does this. But I know that the features keep the NIC card power on while the computer is off, that's why sometime the NIC light is still on at the back of your computer while your computer is off. Because the NIC must be able to recieve this "Magic Packet" from a remote computer to wake it up, it can't do this if it's at the off state.
Maybe a bug in the design, because it shouldn't restart the computer if you are shutting it off. Could be the Operating system itself, a bug within the power management.