The PIN or any other user secret can't decrypt the contents of the flash chip. So turning the device off and removing the flash just makes it infinitely more difficult.
I guess because now you have no way of exploiting any bugs that might've been exploitable. Within 7 days the phone will still try to talk to others connected to its port, after 7 days it'll just charge. I assume that "power off" will also disable data over the port until the phone gets unlocked?
